Catchy in the way a guilty pleasure pop song is.
I spent a while completing everything, but I finally managed to do it. Definitely had fun doing so.
Whacking animals on the head with random objects was never so much fun. Sometimes it can be a bit annoying when it comes to aiming and hitting. It's a little hard to describe in text, but let's just say you don't always bat where you expect it to. Otherwise it is very fluent and responsive, and the fact you can power up a swing when you have nothing better to do is very nice.
The guns are even better, and make the animal rush levels entertaining in a very cruel way. There's no real flaws with those at all. I can't really tell what the upgrades did other than more damage, so I assume it was just that.
The graphics are slightly childish. I might find them cute if these animals weren't your enemies. The 3D-ish feel is cool, but it's _just_ not there. It's like 2½-D. Better than 2D though, and still pleasant to look at.
The audio...is awesome. The shop music is very relaxing while the in-level music is psyching. One time I just turned on the shop music and started doing other things, and I enjoyed it just as much as listening to other "normal" music. The sound effects fit the characters. Bears seem to be both annoyed and in pain when you attack them. The sound of a dead mole/bunny/echidna makes me chuckle. Which sounds very cruel and sick. The low-pitched voice at the start of levels ("Not again") is hilarious as well.
Speaking of moles...hate these things. Oddly enough, if they started stealing your shrooms right away, I wouldn't mind them as much, since then I can just kill them on sight. But the fact that you have to wait for them to come out and play makes them harder to deal with, since you simply don't have the time to sit around and do nothing. Particularly in the last animal rush (which was very good, btw), it's hard to keep in mind that there's a mole about to pop and take one of your ten lives when you're also supposed to be shooting everywhere else.
Having practically infinite money through level grinding and awards...well, it makes the game a lot easier and gives it less of a serious tone when it comes to money management, which I don't think is a bad thing. I prefer this over these games where you have to think over every purchase thrice. Have a game the "infinite" way, and you can always choose to make it harder for yourself. Do it the hard way, and you likely can't make it easier for yourself.
As some others said, I probably won't replay this anytime soon. But the first time was good enough to make up for that.