Hmm, synapseman, perhaps you got a bad copy, because the sample I've tried (and I keep borrowing it from my friend, simply because it's excellent) is sharp at f/2.8. By f/4, it's bleeding sharp. Heh.
TS, it really depends on your needs. IMHO, 50mm to 70mm is one or two steps, but I'd take a constant f/2.8 lens over one that's f/4.5 at the long end. I have this phrase that I always remind my photographer friend with, "a lens is only as fast as its slowest aperture". That means to say, if a lens is a f/2.8-4.5 variable aperture lens, then it's really only as fast as f/4.5, especially since it's at the long end that you want the fast shutter speed.
You may be right, about my copy being a lemon (bought it from Japan, didn't test it out there). Sometimes it could be the camera that plays a part also, I suspect. My Sigma 10-20mm produced terrible shots on my older Konica-Minolta D5D and Sony Alpha A100 (essentially the same camera), but the pics look sharp on my A350/700.
Yup, at the end of the day, the consensus is that it depends on what you generally shoot. For me personally, that 20mm makes quite a bit of difference, and when I shoot weddings, I tend to want to shoot to include the environment, so everything has to be sharp, so smaller aperture is OK. And since Sony is a stabilised system, for indoor night scenes, I can still get by with f/4.5, but with higher ISO.
If I want to manipulate aperture for achieve DOF effects, then I'll use the 50/1.4 or 85/1.4.
When I was doing my own personal research, I came up with the following conclusion:
Tamron 17-50mm
Plus:
Sharp (according to what I've read online)
Constant f/2.8
Minus:
50mm at long end. A bit short, IMHO
Sigma 17-70mm
Plus:
Slightly longer focal length.
Very good close focus capability.
Minus:
f/2.8 only at 17-19mm (approx).
p.s. I have since replaced my Sigma 17-70mm with the Sony Carl Zeiss 16-80mm as my main work lens. But I am definitely keeping the 17-70mm as a very reliable no.2 lens.