The non-VC version of the Tamron is optically comparable to Canon's Mark I 70-200 mm lens. The successor to Tamron's lens not only has VC (vibration compensation, IS in Canon parlance), it also has an ultrasonic motor.I've seen comparisons, and the Tamron doesn't touch the Canon.
Also dxomark puts the Tamron squarely in the same league as both Canon lenses. Resolution-wise, it sits right between the Mark I and Mark II. The Tamron even has the highest final score -- although I'd be very reluctant to boil down optical performance to one single number. There are other aspects (AF speed, efficacy of the image stabilization, quality control) which are not reflected in the score.
Photozone, ranks the non-VC predecessor lower than the Mark I and Mark II, but they have yet to test the successor.
Before people here start a brouhaha, I'm not arguing the Tamron is better than the Canon's (that most likely depends on the definition of »better«), I'm merely refuting the claim that Canon's lenses are playing in an entirely different league than anyone else. Arguably, the Tamron has better price-performance (even though the VC version is markedly more expensive than its predecessor). They are not. Given the right budget, any of the big lens manufacturers can produce a great lens. There is no magic Leica or Canon L pixie dusts that circumvents the laws of physics and economics. Sigma and Tamron have been moving to the higher end of the market, the lenses they have released have higher price points than their predecessors.
Unless your needs are very particular and/or you are independently wealthy, it's usually much better to go for a cheaper option and a second lens/budget for other things rather than scraping every penny you own because of some perceived deal breaker a Mark I/third-party lens has over a Mark II.