I'd consider the Canon EF 70-300mm f/4-5.6 IS USM as an alternative. It's not one of the "through the roof" expensive tele zooms, but it does have some of the nicer features, esp image stabilization and a *relatively* fast zoom.
I looked at the two lenses myself (sigma, canon), but after trying the Canon in person, decided it was worth the extra money. For me, a big decision maker/breaker was the IS since I wanted to take a lot of photos in Alaska (wildlife, some shot from a boat on a day cruise) and it made a big difference. The boat vibrates a lot, so the IS was a must-have. For wildlife, it makes a noticeable difference at 300mm.
My dad had his 28-300 Tamron with him, and we swapped them back and forth (we're men, we have to see whose is bigger and better) and it was nearly impossible for me to get clear shots on the boat with his lens, and it was about a 2 stop improvement for wildlife shots (not on the boat) at 300mm. I'm guessing the Tamron was a reasonable approximation of the Sigma, although I realize that's not an ideal comparison.
As far as clarity goes, I think the Sigma is pretty good for the $$. I tend to think they're the best entry-level lenses out there (at least for some of their lenses) so this is not to disrespect them in any way. But the Canon has more functionality and should run you about $550 on Amazon or from
B&H or
Adorama.
Again, just my two cents, but if I'd gotten the 70-300 from Sigma or Tamron I'd have felt a little bad about it when I could drop another $300 or so for a lens than I like a lot better. If you can't afford the Canon, you will be OK with the Sigma, it's a decent lens.