I have several Tamron lenses, but on Nikon bodies. I have not had any AF issues, especially in good light, beyond what one would expect. Carefully look at your out of focus images to see if any part of them are in focus. That might mean your AF system is locking on something you did not intend. Is your AF system locking? On Nikon's a little dot shows up in the lower left hand corner of the viewfinder when the AF system locks in on the focus point. I'm sure your Canon has something similar. If the dot flickers, or the dreaded left and right arrows appear, the camera is having trouble locking on the focus point. Is your AF system hunting? Camera phase detect AF systems perform better when they have a high contrast focus point, so it's a good idea to focus on an edge if you can find one. Your AF settings are important. When using multiple point AF modes, the AF systems tend to focus on the part of your subject that is closest to the lens, but if they lose lock, they can focus on the background. The first thing I would recommend is trying some different AF settings like go to single point AF and AF continuous. As others have mentioned, your technique is critical with long lenses. Any camera shake will cause blur, so it's a good idea to up the shutter speed to at least 1/focal length, but for testing purposes, try 1/1000 and see if that helps. Is the VC switch on your lens set to on?
I have gotten very good results with Tamron glass. I hope you get it figured out. Good luck!
