I have this lens. My experience is full of contradictions.
I purchased this lens in February based on reviews when I owned only 650D with kit lens 18-135 IS STM (Rebel T4i). I wanted a standard zoom to slowly prepare for purchase of 6D. Initial experience was excellent. Better image quality, better colors, better DOF ... Two weeks after the purchase I noticed that my battery in the camera gets discharging even when the camera was turned off. I found that this was a common issue of the first generation when used with newer Canon bodies (650D and 6D). I took the lens back to retailer and I made a serious mistake. I let them to repair the lens because I wanted to keep it instead of requiring money back. Retailer first didn't want to believe that such problem is even possible but I referenced several reports from Internet. It took 4 weeks to get my lens back. It was "repaired" in local service center. Repair report scared me - VC was replaced and CPU reprogrammed but the issue disappeared.
I used the lens with my 650D quite happily since that. Some of my images had a really bad focus but I thought it was my mistake (I started with photography in November 2012). Two weeks ago I bought 6D. While it make my 70-300L even much better lens than it was with 650D it also made Tamron completely useless with aperture less than 5.6 or 8 (depending on distance). My Tamron has a terrible back focus on the longer end. When I made a test myself it had 10-12 cm back focus when shoting from 1.5m distance wide open. FoCal was not able to fix this issue. It predicted that necessary AFMA setting would be at least -36.
I don't understand how could I not notice this on 650D. Somehow I believe that it is not pure back focus issue because in such case all of my photos taken with 650D wide open would have to be out of focus but only some of them really are. It looks like a serious AF inconsistency. I took the lens to service center on Monday. I skipped the retailer to make this quicker. When I saw the service center (the only one authorized for warranty Tamron repairs in my country) it just made my frustration bigger. Somehow I don't trust them and I really don't believe that they are able to make any serious repairs of such complicated thing correctly (understand reliably in the long term). I will see. I'm also very curious if they can fix the problem in claimed "3 days" promoted in the whole Europe.
While I still believe that this can be a wonderful lens my copy is definitely not that one and I don't believe that just fixes in local service center will change that. I live in a small country where only few Tamron lenses are sold and the support is equivalent to the size of the market. It also means that available lenses are probably all from the first bad series. I know another owner who uses this lens on 5DII and he also complains problems with inconsistent focus.
My another problem is with VC. I'm not able to get what is claimed in tech specs. 3 stops? No way. My real world experience is 1/30s shutter speed on 70mm to get a sharp image. When I go with 1/20s it will be blurred when 1:1 zoomed. That is more like 1.3 stop but I have similar problem with all my lenses so it can be just in my technique.
My opinion about Tamron is so far: You get what you have paid for. You can get a great lens for a great value if you are ready to suffer initial fight to get a good copy but you must live in the big market handled directly by Tamron because otherwise you can have a hard time to get replacement easily (my case). It is lottery. I personally don't like to be QA/QC, especially if I pay for that.
Canon's MkII costs 105% more in my country and it lacks IS. It is a big price difference but if it saves you from this madness it probably worth the money. I would never admit it before but my problems and big satisfaction with 70-300L (even used!) changed my mind.
We have a proverb: "I'm not so rich to buy a cheap product". More and more I think if this wasn't the case? I'm deciding to go a hard way and try to use warranty to get my money back because my lens doesn't meet specs. If I get money back, sell 650D and its kit lens, I will need just few hundred bucks more to get Canon's MkII. I will miss VC a lot but it is just only 1.3 stop at the moment. On the other hand I wanted to use money from 650D and kit lens to acquire 100L.
I will see how my lens performs once it is returned from the warranty repair. Somehow the issue described by Roger from LensRentals would make my decision (and warranty claim for money back) much simpler.
Just to continue my experience ...
I got the lens back on Thursday (hey really made it within 3 day). I used FoCal on Friday to check AF calibration. It didn't need any AFMA change on wide end but it needed -13 on telephoto end! That a big disappointment. I gave both lens and camera body to service center and they returned me a "calibrated" lens which still needs -13 AFMA which I must do myself. At least this time the required value was within available range. This change increased quality of focus on "calibrated" lens (according to FoCal units) by 100%. My problem copy is not as sharp as Dustin's copy but it is now satisfying for my needs.