Didn't like the first one that much, it was just like a crappier (yet more explicit) version of the "crusader" series...but then, I never really got into it...
I played the second one, and the expansion pack "apocalypse weekend".
Both fun outlets to just run around killing and killing and killing (I liked pissing on peoples faces, then dousing then and setting them alight while they were vomiting, so they run around still hurling, on fire.), but the level design was REALLY shithouse, and the game was just a chore to finish - but...the expansion pack felt a lot more like a "real" game, better level and mission structure, some cooler weapons, bosses, and some FINE zombies, better atmosphere too
If you're interested in the series, perhaps you'll like this:
http://www.gopostal.com/store/fudgepack.php
Looks like great value there, I'm grabbing it myself.