I'm finding the focusing a little bit hit or miss, and to be honest
I'm not knocking your photography skills in the slightest but I was thinking the same about this lens.
On the London meet today, I was struggling to get a decent shot in focus @ f/2.8 in a dimly lite section in a pub.
I knew in AV mode that the shutter speed was fine so it wasn't a case of being to slow for me to take the shots hand held even with IS.
I just thought it was down to hit and miss with the lens focusing.
I asked one of the more experienced Canon users (ramaran) to take a shot with my kit and asked him to use the settings that he would normally use.
He took a few shots that were very sharp and nothing like my dire efforts.
The only differences he made was to up the ISO where I had it set to auto and raised the exposure level.
Talking to him made me realise that the problem was my technique / skill rather than the lens.
I did have a few doubts about this lens even though I know I've taken shots of my dog in the house @ f/2.8 that are probably my sharpest yet.
Learning how to use this lens in even lower light has been a valuable lesson.
Overall I think that provding you do get a good copy of this lens (I think Tamron will sort this issue out quite quickly), you'd be more than happy with it.
I agree with those who regard the price of the Canon is very high for a non L lens even though many claim the IQ is of L quality but I would want L build quality for the price.
I woud have also been happy to spend £500 on one as well.
If money is no object and you can afford the Canon then fine, but if I believe the Tamron is better value for money.