At first I thought it was really weird (before I owned one, from just trying it in the apple store).... but, after I bought my iphone3g I got used to the keyboard within about 30 minutes or so.
Now, I couldn't be faster. I had a TytnII otherwise known as an At&t Tilt with a full keyboard, and i type much faster with my iphone than I ever could have or did with the Tilt.
So, yes.... I got used to the iphone keyboard and love it.
Best thing about it, is that I don't have to apply pressure to type.... unlike manual keyboards.