I just got a Sigma 15-30 today and despite some pretty good reviews over on FM, I was prepared to be underwhelmed due to the low price tag. Not being a landscape shooter, or even really a WA guy at all, I just needed something cheap for those times a WA was called for. Well, was I ever wrong!!! Might just be a good copy but this thing is VERY sharp edge to edge at 15mm and not bad at 30. That's wide open. Stopping it down to even 5.6 and its fantastic. Colors are nice (not the normal Sigma warm) and the contrast is good. Obviously there's barrel distortion and there is a tiny bit of CA on extremely high contrast shots but its nothing you wouldn't see in most UWA lenses. The AF is also a bit on the slower side since its not HSM but most aren't buying UWA lenses for speed, at leat I wasn't.
Truth be told, its on par with any of the wide to normal L zooms I've owned and I am more than pleasantly surprised at just how good this thing is. And to think I'd almost written it off before even getting it! For $300, I really don't think you can do much better, if at all.