Blowing up zombies is fun while it lasts.
The unlockables are all very simple to get. All of the upgrades all come at set points in the game. You unlock new avatars the same way with the exception of some of the secret ones. None of them take much effort save for finding a special point for Slipknot and Chimaira.
In the end, it's fun but it's not going to last you very long. After I beat every mission it only read that I had played about 2 hours and 10 minutes. I know I spent more in it but maybe it only counts your best run on each mission. Of the 35 missions only 7 in my personal copy are missing their gold medals so that may give you an idea of how quick and easy it may be. There are 3 difficulty levels to pick from if you feel the need. I can't say I ever changed mine though.
I can't say anything about the multiplayer modes because I couldn't find anyone to play with. Sorry.
You can probably find Infected for less than $15 if you look somewhere like Circuit City (I believe I got mine for about $10 there on Black Friday). It's certainly worth picking up if you've got a little cash to spend and don't mind a short story mode.