TL;DR: If you're looking for a fun, quick, casual music "game" it's worth it. If you're looking for a more serious music game or sequencer, move along for now.
Basically, you're given the option of a Story Mode or Quickplay.
Story Mode: You're taken on a short journey in a futuristic cyberworld thats sweet jams have been taken over by a buzzkill. Your quest is to repair 5 "Jamtennas" in order to bring bands and groups of people (and one puppy) back together, then take the city back. Each Jamtenna consists of 4 or 5 segments that tell a story, and each segment forms a different part of the song. Each part of the song gives you 4 or 5 different instruments, a background track, and a 2-bar loop to program in. Each instrument gives you 4 different pitches (or sounds depending on the instrument). Once the Jamtenna has been repaired, the game then crafts your segments into one sweet song and gives you the option to export to MP3.
Quick Play: You're randomly given 4 or 5 different instruments and a shuffle button to swap all of the instruments (there is currently no way to manually select the instruments, or the hold onto the ones you don't want to shuffle in). You're then given a background track selected at random, and are allowed to go to town on it. I haven't spent much time with this feature yet.
All in all, it's currently a super basic music sequencer with a cute story attached to it. If you only do one playthrough, you could probably beat it in about 1 or 2 hours, and other than the patterns you throw down, you can't really deviate from what you're given (in terms of notes and instruments).
As far as I can tell from reading the discussion, there's a lot more coming with this game as the developer works on it. MTCS is a pretty solid guy - he's very active in the discussion boards and listening to what we all have to say.
As stated in the tl;dr: If you're looking for a fun, quick, casual music "game" it's worth it. If you're looking for a more serious music game or sequencer, move along for now.