If you really like the game, why not? I'm sure there's more than one person that's dumping alot of money into their PC as a not-so-indirect relation to Doom 3.
I paid like $15 or whatever it cost to be in the SWG beta. That also lasted 2 months and would've cost twice that if the game had actually been out, so...
CS is free, but CS:CZ is not. They're the same game, only CZ has a single player mode and will allow you to play the beta of CS Source. The standard free download does not.