I would second the opinion on the Ed Mika adapters, but it does need to be for something worthwhile, whatever you decide that is.
I use a T3i, and am looking to get a decent Canon long zoom, maybe the 100 -400 when the new one comes out. In the meantime I decided to pick up a used Canon FD 500mm reflex (I used to own one that went away with the rest of my old gear when my house got burgled) for an inexpensive long lens capability. There are lots of pros and cons with reflex lenses, but for around $450 I got the lens and the Ed Mika adapter. The following pictures I took within three hours of the adapter arriving through my mail box. I am very pleased with the result. Any limitations on quality are probably the nature of reflex lenses, they are strongly disliked by some, but it is very light weight, and all pictures were hand held giving an indication of its versatility.
Choose your lens according to your taste, but if you have one you wish to use I'm not aware that you can do better than one of Ed's adapters.