Preliminary findings and observations. A link to the images is found near the bottom of the page or just click here if you aren't interested in the comments. I am not a pixel-peeper and don't do charts etc. so lab results might differ.

Pros:

• Remarkably sharp (sometimes excellent) over much of the FL range on a DX body and holds up well  into the corners.  Images still  sharp at 100% which takes some doing.
  
• Good contrast/saturation. Consistently vivid and snappy indicating resistance to flare.
• No chromatic aberration worth mentioning - quite amazing. APO means something after-all. The first hints show up at about 120% which is irrelevant.
  
• Fast, accurate AF-S (HSM) (although the D300 probably deserves most of the credit) and it works remarkably well even in poor light with this slow lens. Nice for nature shots in Continuous mode with tracking on the D300 owing to speed of the HSM.
  
• Exceptionally effective dual-mode stabilization (OS) but see later caveats. Sharp handheld shots at 500mm with low shutter speeds readily achieved with practice and care. Very nice for the wildlife photographer wanting to get away without a tripod.
  
• Nicely thought-out mechanical design. Excellent grip/tripod mount, zoom lock (but only in collapsed configuration) etc. Does not sag or wobble at full extension. Nice grips on rings. Hood and rear cap are better than Nikon designs. Not that it's relevant to performance but has a handsome appearance with black matte finish and highly visible markings. Looks like metal. Perceived build quality seems excellent but do not consider myself very qualified to comment on this. It certainly feels and looks solid.
  
• Sensible zoom range is not so great it threatens image compromise in the design. I mistrust aggressive zoom ratios. To me, 150 - 500 mm is perfect for wildlife. For shorter focal lengths - scenery and portrait work -  smaller and lighter Nikkors are there to do the job.
  
• Very smooth manual focus ring which engages automatically just like an AF-S Nikkor.
  
• Somewhat stiff zoom ring which I consider a "pro" because it discourages creep while shooting at an angle. Some may not like this so - to each his own.
  
• Fully Nikon compatible - reports everything to the camera and generally behaves like a Nikkor.
  
• A whopping 10 year warranty. Now there's confidence for you.
  
• Excellent padded travelling case with high density foam internal bolsters and strap. You'd probably have to pay $80 for this on its own.

Cons:

• It's pretty darn slow being F5 -F6. This is the biggest trade-off leading to low cost. But if it were F2.8 it would cost $10,000. You can't have it both ways. The rather small maximum aperture limits focusing speed in poor light and may require extra care with focusing at long range.
  
• The manual warns against dismounting the lens with OS and camera powered up as that may result in lens damage. This is somewhat worrisome. Let's hope they don't really mean it. Very easy to forget because with Nikkors it doesn't matter. This has to be an engineering oversight.  I put a sticky fluorescent label on mine with a warning and so far no goof-ups.
  
• OS takes about 2 seconds to lock-in after pressing shutter release half-way whereas Nikon VR is instantaneous. You will certainly get fuzzy images (like my first dozen attempts) if you are in the habit of punching the button at all (Nikon spoils you with their exceptional responsiveness). The camera will signal ready (on basis of AF lock) before OS settles-in and you can readily beat the OS. Not a flaw so much as a peculiarity you must get used to. The OS makes a small grunting sound before and after engaging and this is a big help once you learn to listen for it. As long as shutter button is pressed half-way OS remains engaged. Once engaged, OS is rather noisy with a whine of varying pitch related to camera motion. Doesn't bother me in the least but in quiet environments some nit-pickers might find it distracting.
  
• Not seriously included at all as a "con" but weight and bulk are considerable. If you want "reach", you have to tote heavy iron, no matter who makes it. Just be warned this lens is no exception. At least you get that great carrying case.
  
• New product so long-term reliability (the excellent warranty notwithstanding) is an unknown factor.

Bokeh ... not sure what to say about this yet one way or the other. The out-of-focus darker areas seem to blur very nicely but there are brighter spots once in awhile where a faint outline of the 9-bladed diaphragm is evident so it is by no means perfect. Images recently posted (2008/08/20) may provide some help. Looks OK (at least) to me but will depend on your needs. Zooms generally have mediocre bokeh.

Recently (early 2012) Thom Hogan has pretty much trashed this lens rating it only "good" from 200 mm to 400 mm and not so good elsewhere. I usually agree very closely with his findings so this is all a bit odd, especially as the divergence in opinion is so large. My results are more like those enthusiastically (has he any other way?) reported by Ken Rockwell. I am sticking by my generally positive assessment which applies throughout most of the focal length range (on DX bodies at least).  My only real issue with this lens remains that it is quite slow. What may be at work is unit to unit variation in quality for which "brand X" lens makers sometimes take a deserved beating. It is possible I got one of the good ones and Hogan did not. It is also possible he did not use the lens long enough to discover that stabilization takes so long to kick-in. It needs at least 2 seconds. In any event, you should read his review. IMO this lens still deserves a fair try-out by anyone wanting a long tele-zoom giving impressive results (??) without the big price tag. Get it from a dealer who will agree to take it back if you don't like it and test carefully before deciding to keep. Remember that it is notoriously difficult to get consistently good results even from the best long lenses. You might want to check my article on that subject in "General Topics in Photography".

 
Conclusion:
 
Very good - at times excellent - image quality throughout. Images suitable for large prints most of the time. Its only real weakness is small minimum aperture but if you don't want to pay $5000 for the Nikkor 200 - 400 mm, that's the tradeoff you must accept. Despite being an F/5, auto-focus is fast and accurate in most light using the D300. Softens just a bit approaching 500 mm but still very good even for large prints - a welcome surprise. Absence of CA is a real pleasure.
 
"Value" is a poor criterion to use in grading a lens although this unit certainly delivers it if that's important to you. Even if a product were free why would you want to have it if it is crappy? Either a lens is good or it is not. To be sure, an exotic Nikkor will out perform the Sigma but will you be able to see the difference in the sorts of pictures you plan taking? Many owners will be working in good light,  stopped down to where diffraction limits acuity anyway but if your need is, indeed, for large aperture, fast shooting then by all means look elsewhere.
 
This is a fine lens and I am not adding "for the money" although it's that too. The low price is merely the icing on the cake.

Sigma suggests their teleconverter for this unit. The lens may be sharp enough to withstand a TC (providing it is a good one) but losing at least another f-stop puts you at F/8 or a bit slower.  Goodbye AF. This does not seem a very wise or useful option at all although for wildlife at close range, within reach of the SB-800 flash unit, might prove acceptable.

Demo Image Comments:

All pictures taken handheld and rendered from NEF using Silkypix Developer. The presentation is provided using JAlbum and hosted by my own web site. Images re-sampled to 100 dpi with 20% JPEG compression using Paintshop Pro X2. At the bottom of each image you will find EXIF information and a Help link for using JAlbum. You can run the demo as a slide-show or click on the images to view individually (or use the back/forward arrows in JAlbum).

It goes almost without saying these compressed and resampled images lose significant detail but when viewing on a monitor you are stuck with 100 dpi anyway.

• Utility transformer shots taken at range of 120 ft., 500 mm. The detail crop clearly resolves threads in the bolt near edge of the frame. This image, the fuse assembly and the truck are uncropped.
• On truck shot, note grill detail holding up right at the edge of the frame and no hint of CA in the chrome flashes. A crop of the vehicle trim is crisp and vivid.
• The moon crop will typically appear at a bit greater than 100% on most monitors - a rather ridiculous magnification for realistic viewing but the image is still impressive.
• The House Finch (first one) was at a range of 30 ft. with focus on the eye. Second finch at about 60 ft using SB-800 flash (windy, subject moving).
• EXIF lost for some images. For those undocumented, all were taken at 450 - 500 mm and at range < 40 ft.

If you have comments, please post them in the Nikon forum where you found the invitation to visit here. I hope you will find some of this interesting and even useful ;-)

Click Here to open the demo.

(The demo will open a new instance of your browser.)

More to come in the next few days if there is any interest.