As usual, I'll give my opinion, and, after that, get flamed...
But I do think it could be useful, since I've owned two 28-70 f/2,8.
-Sure, they look nice if you look at the specs (specially the 2.8), but, since they're (according to actual owners of the current 24-70) a little on the soft side at 2,8, you wouldn't benefit from that. So don't fall for the 2,8, since your practical quality aperture would be limited to 3,5. Aperture isn't everything.
-They aren't very well built: too much low quality plastic, and their fragile "pro-finish" only looks good in the pictures.
-They're VERY prone to get fungi. I really don't know if it's because the sealing is worse (if there is any), or because they lack a coating, but I've had fungi problems on 3 out of the 4 Sigma lenses I've had (one of the 28-70, a 105 Macro, and a 70-300), while my Nikkors, kept under the exact same conditions, still look like new.
- The Nikons were very Noisy, I don't know about the Sigmas for Canon.
- They were too heavy and big.
On the contrary, I've heard and read many good things about the Tokina.
In my opinion I think you'd be better getting:
- A good non-2,8 zoom (if you don't own one already)
- 50mm 1,8 ($84) for low light/small DOF
or, if you already have the zoom, get
- A 28 f/2,8 ($169.95) or a 24 f/2,8 ($280)
- A 50mm f/1,8 ($84)
and eventually, add later to that a second hand 80 f/1,8. ($250)
Any of these two alternatives will prove to be better quality wise than the Sigma, better for low light, smaller and lighter. Save yourself money and get a Canon (or Nikon) from the beginning.
