I've only taught off the iPad for a couple of weeks, but I love it!
I actually think it's easier than preaching off paper notes. The reason being, there's no light directly over me when I'm preaching, so occasionally I'll get shadows on my notes which make them a challenge to read.
With iPad, the lit screen gives me all the light I need to read all of my notes.
The other question people have asked is, "What software are you using?"
I am using Pages for my notes. I write my messages in Pages on my Mac, so transferring the notes into the same program seemed only natural. Plus, I can edit the notes once they're on the iPad if I need to.
Bottom line: I highly recommend teaching with the iPad. It's an elegant device and that serves a lot of purposes and preaching is one of them.