From all accounts, the picture quality is good, the lens seems well-made, and it focuses fast on static objects (the AI servo mode on moving objects is still an open question, being discussed on fredmiranda.com). The lens doesn't feel heavier than the Canon 70-200mm f/2.8, though it actually is several hundred grams more massive.
Conclusion: The value for the price (e.g., compare with the Canon 100-400mm) for this lens is good. Tamron likely has a winner on its hands.
PS: with certain Canon models, AI servo apparently does not work well (details in this fredmiranda.com thread)