Okay, I actually own the new Tamron 70-300 IS.
I've only had it for about a week, so can't give a real thorough evaluation, but so far I am pleased. I was not expecting it to be of the same quality as the new Canon "L" 70-300 but I did want a sharp lens with a bit more reach than the 55-250 mm EF-S.
Anyone familiar with the 55-250 mm knows that it is an extremely sharp lens and with IS it remains one of the best values available. The biggest problem with the 55-250mm is the build quality (or at least the perceived build quality). It's light and feels plastic-y, although I have to say I've been carrying one around for the past 2 1/2 years and never had a problem with it.
When considering the Tamron, my questions were:
- Would it be as sharp or sharper than the Canon?
- Would the IS be as good or better than the Canon?
- Would the build quality be better than the EF-S lens?
- Would the extra 50mm reach be significant?
So far, all the answers have been "yes."
- It is at least as sharp throughout its range as the 55-250 is throughout its range (which is very sharp);
- It's a little premature to tell, but so far the IS seems to turn in results that are at least as good as the Canon and maybe better;
- The build quality "feels" better. But, again, only time will tell. It is a much heavier lens overall and feels solid. (In fact, because it is so much more robust than the 55-250mm I'm actually thinking about keeping the Canon for times when I want a lighter lens);
- The extra 50mm reach really is significant. Of course, on a 7D it's the equivalent of about 75mm.
If I had unlimited resources, would I have gone with the "L" lens? Maybe, but that's academic. For my purposes, I decided this lens was worth trying. I did not want to invest $1,500 in a lens when the truth is, the telephoto zoom I really want needs to be at least 400mm. I can use this lens most of the time, rent a longer lens for travel or special occasions and either wait until Canon or Tokina finally comes out with their long-promised 400mm zooms or Sigma improves on their 500mm zoom.
Some people need or want the "L" version. It's a personal decision. Everyone needs to decide for themselves. Dilbert seems to have decided the incremental value isn't worth the added cost. Neuro decided it was. Neither one is right or wrong, just different viewpoints.