It will work on your 7D without problems at all, however if you are buying older used version keep an eye on them to make sure they are as recent a release as possible. Whilst I'm not aware of any issues with this lens, remember that Tamron, like many others, reverse designs their lenses to fit canon. This means small changes in teh camera/lens communication method can break down on older 3rd party lenses; sigma will rechip (for free) many of their older lenses, although there is a cutoff period for each lens, beyond which they no longer support them.
Like I said, I'm not aware of any problems at all with the 7D specifically and I've no reason to think otherwise that, in the right hands and the right light, it would be a fantastic choice. Indeed the 90mm is a very popular choice within the budget macro community, offering a good price, construction and also a great image quality for its price (in general almost all the macro prime lenses are on the same shelf with regard to image quality).
