I've found that most work very well. The 3rd party brands sometimes have issues. I would highly recommend one do a search on dpreview.com or fredmiranda.com for your lens and compatability with your lens. There have been a few minor problems. Some, lose functionality in their autofocus. But for the most part, dslr's are made for the film lenses. As Elkman pointed out, the 1.6x frame factor you get with the new smaller digital sensors gets you a lot of lens for your money if you are shooting wildlife with telephotos. There are problems with wide angle lenses, but they have fixed that with affordable lenses down to 17mm now. I actually found that the autofocus worked much faster on my old Tokina lens and my new 40D than it ever did on a canon film camera. Additionally, this lens was often soft at the edges with a film camera that used the whole lens. Now that my dslr uses less of the lens because of this 1.6x crop factor, the edges of my photos on this lens are much sharper than they ever were with film cameras. The dslr's just use more of the center sweet spot of the lens.
shawn
