REALLY WIDE AND REALLY SHARP but wide open is only f/4 at best

I've been using the 10-20mm HSM Nikon mount version of the Sigma super wide zoom and really have come to treat it as my favorite lens. Here's why:

1) The wide end at 10mm is really extraordinary, and if there is a slight degradation of quality in the corners, you'll normally never see it unless doing exhibition prints.
2) The autofocus is remarkably good for such a slow lens (the autofocus device on a camera like the Nikon DSLRs requires the camera to read the contrast of the subject through the lens, and a brighter images really helps in low light). So kudos to Sigma for that
3) And the reason is probably because the contrast is apparently very high--this is a product of lens design and coatings, and gives the images an appearance of internal snap (something slightly different than resolving power, which is very good on this lens but not like the Nikon 14-24mm masterpiece).
4) It feels really good mechanically. No compromise here. To admit something along these lines: I dropped it recently (and the camera too) and it bounced around the sidewalk and both the D40 and the Sigma were fine. Very very solid stuff.

Drawbacks:
1) The wide open aperture is only f/4 at best--once you zoom in to 15 or 20mm, you are at something even "darker" like f/5.6 at the other end, which is slow! The competition isn't much better, unless, again, you compare to the Nikon 14-24, which is not only expensive but huge. And it doesn't go down to 10mm.
2) The sharpness is excellent FOR THIS KIND OF WIDE ZOOM. It will knock most viewer off their feet. It will come in at very good to excellent in lens test in a laboratory, but nothing legendary. That's pretty much what you expect.

I shoot a lot at night, and with a 10mm setting you can shoot at 1/8 second without noticeable shake (I concentrate on holding things still). Maybe they'll include VR or IS technology someday. That would make this lens indispensible. As it is, it's mere wonderful. Really. I wouldn't hesitate. For serious hard core pros with specific requirements, a few test shots are in order. For the rest of us, shoot and be thrilled.