Combat Baby is a nifty tune. What makes it enjoyable as opposed to boring (unlike a lot of the on-disc warmup keys songs) is that the song is a) short and b) lacks huge pauses in the action; the keys start at the first chorus, pause for thirty seconds or so, then come back for the rest of the song.
The chorus riff is a simple sustain part; four single notes held down for a few seconds each. Not boring, since the song is fast paced.
But after that? The song throws at you one of the most enjoyable keyboard parts in the entire game: a moderately speedy riff that snakes up and down the keyboard in a super fun manner, followed by some simple, but fun two note chords.
All in all, it's really fun, and is weighed down to a four-star rating for purely technical reasons. Super-enjoyable.