I'm downloading this now and I've waited a long time for this game. The original was one of the all-time greats and so far the reviews indicate this "prequel" is worthy of the original.
I've enjoyed it quite a bit so far. Adapting to the stealth thing is somewhat annoying as someone who prefers to run and gun (Serious Sam 3: BFE :D) but after adapting I like it. (Also, single-player game not ruined by multi-player crap is great)
I've been awfully tempted to get it. I've been hearing a lot of good things about the game. Supposedly there may be some DLC being announced soon to.
It's not without it's flaws (hello stupid bossfights), but it's worthy of the original and it's been great fun so far. If they fix a few things have put some DLC content on, then it will be a really great game.
This is on sale at Amazon right now for $10. It's practically a steal at this price. This is a steamworks title, so it can be activated on Steam. Just get the cd-key from it, and input it into steam.