We had a look at it, but the setup was a right pain in the ass, and it offered less features than the latest phpBB versions. Tempus and I have worked with phpBB a lot now, so we know what we're doing with it and can produce a much better forum with phpBB than we could with MyBB (due to it being new).
Looking clean is 100% based off the design used with the software, and not a MyBB versus phpBB thing.