Um Jammer Lammy certainly was a change from Parappa the Rapper - no longer did you have to follow the raps, but rather mimic guitar riffs. Personally, I like Lammy quite a bit better in this respect. Even though the songs weren't quite as catchy, I still liked the guitar sound of the game and decided it would be cool to pick up the soundtrack.
First things first: this is not a direct OST like the original Parappa soundtrack. Many of the songs are sung either entirely by Milkcan (sounds like Katy Kat, for those familiar with the game) or in a duet with the song's original instructor. This isn't necessarily worse or better - it's just different. After listening to this CD for awhile, I kind of like Milkcan's version better than the songs in the game, though they do take a bit of adjustment. One of my personal favorite songs is "GOT TO MOVE! (Millennium Girl)", in which Parappa even joins in halfway through the song.
Katy Kat doesn't exactly have the best voice on the face of the planet (there are a few lines she sounds pretty bad on) but since Milkcan is supposed to be a "girl next door" band, I guess this can be acceptable. Still, the lyrics are kind of bizarre ("There's no foolin' around with deers"? "Did I eat my drink"?) and no amount of changing could save Teriyaki Yoko's wretched song (though it has been arranged and lengthened a bit for this CD).
There are also a few other extras bits on this CD: an instrumental "Theme of Rammy", which is a slow, downbeat guitar piece that appears momentarily in the game, and a few of the "cutscene" songs from the game, also sung by Milkcan (such as the ridiculously goofy country song "Casino in my Hair").
The biggest flaw with this CD though is the length. Only 33 minutes? With that amount of space, why not put on the original versions of the songs? Heck, why not throw the Parappa mixes of each song on there as well (even though I never cared for them personally)? And worse yet, *where* is the Master Onion song from the first stage of the game???
Fans of Um Jammer Lammy will most likely appreciate the new versions of the songs. However, because it is so short and fairly expensive, I'd really only recommend picking up Milkcan: Make It Sweet if you can find it for a decent price.