I say buy it. I never played it myself, but if it's better than the first one(and most people say it is) and given how many good mods there are for it, then it really is worth the 24$ .
.. . You've nearly convinced me sir... Nearly :D
Personally, I didn't buy it because I'd be paying $24 for a DRM-free game, for a single mod that I might not like because of the horrible people that play said mod, that I can't play with my girlfriend because it requires a key that you only get one of to be put into an online account to play online.
Part of the reason I love GOG and its DRM-free games is that they're DRM-free. I can treat them like console games, in that I can share them with my girlfriend and we can play together.
EDIT: Some of them even come from the forbidden era of "hotseat modes". Seriously, what happened to sharing a gaming machine. Even games on consoles are moving away from the whole split-screen thing.
It's awesome that Serious Sam 3 supports 4 player split-screen even on PC. Or at least that's what I've read. Haven't had time to download it. If it doesn't, I'ma be pissed. :P
Yes, I too would like to play with my gf. If I can only play it through one "account" that is really gonna blow.