You might like it, you might not: I did.
While I normally shun such categories of 'casual' and 'hardcore', this is the perfect example of an extremely difficult game wrapped in the most unassuming of packages. The musically gifted may not find too much trouble in the 50 mini-games of Rhythm Heaven, but as I found out when letting other people play, those without a strong sense of rhythm will struggle mightily with this game.
Speaking of the mini-games, each has its own song and actions that must be performed in time with the music. There are only three types of actions: tap, tap and hold, and flick. However, the use of the three actions is quite creative and doesn't feel stale. The music is also quite catchy and of surprisingly high quality, although some may begin to grind on your nerves. Also, be warned: the embarrassingly stupid lyrics to some of the songs may get stuck in your head and you may find yourself singing "Love you love you love youuuuuuuuuuuu... More than yesterday. I suppose!" at some very inappropriate times.
The visual style takes some obvious cues from WarioWare, and it is quite attractive, although the style will most certainly not appeal to everyone.
I classified this as a 'rent it first' title for a couple reasons. First of all, you may simply not like it. It is not a game for everyone, and nor is it meant to be. Secondly, if you don't like chasing the perfects or medals and unlocking things, you may find yourself finished with the game in less time than would warrant a purchase.