of those which will work with it, focus should work in a normal manner. Not all Canon lenses will work with the extender. My 2x works fine with a 70-200 f2.8L IS which is where I use it.
I have another quick question. i recently purchased a 2x teleconverter (made by Canon) to use with my 40D. Are there any tips or tricks to use with this? I am getting a good zoom when I look through the lens, but my pictures are blurred.