There's some good variety in here. Some of the songs are a bit muzaky, but generally I like them. I'd say they are grade 5 or 6, with a pretty high impressiveness-to-difficulty ratio. The main difficulty is really in the complicated rhythms (the included CD helps a lot here if you're feeling lazy!).
I really think all sheet music books should let you preview the audio, iTunes-style. It would make choosing books easier.