I wanted to pick it up once the price dropped, but I thought most people would have stopped playing it by now. Probably boring as hell if you play it alone.
I picked it up for $20 a couple of months back. I played the entire game with a friend who also had recently picked it up. It was mainly us two for most of the game, and it was fun. I agree with the other posters that solo is not as fun.
You can join other random people's sessions at any time, but the problem with that is that this game has been modded to shyt by a lot of players. You'll run into people who have incredibly overpowered weapons that kill any zombie in one hit, and it's no fun when you cant even get a chance to attack a zombie. You can also get the modded weapons yourself if you wanted. Other players will sometimes offer to give you some (you can trade weapons in-game). I chose not to because I wanted to play it legit.
Another problem with randoms is that people have created weapons that they can team-kill with, and they'll sometimes join sessions to simply grief other players. When you die, you are penalized 10% of your money, so this becomes really annoying.
That being said, if you find somebody who is willing to play the game legitimately, add them as a friend. You can make your sessions private so random a$$holes cant join and fukk up your game.
It's worth $20. Once you are done with it you can trade it in for $10-$15. I got a $16.00 trade-in credit from Amazon for it less than two weeks ago.