I try Gambit Scheme almost once a year, therefore I am a casual user at best. It may be because Racket comes with a fullfilling environment and a book. But now with Gambit REPL on the iPad, this is going to be more frequent. It reminded me the days of LispMe and the Palm IIIx years ago.
Update: A few days later I wondered whether there exists a good calculator for the iPad. And then it struck me: I already had a Polish Notation one: The Gambit REPL.