Apocalypse Weekend is much better than Postal 2, but it still has plenty of problems.
You start off in a hospital after being shot in the head by your special someone, your doctor is telling you a bunch of useless mumbo jumbo that you end up ignoring anyway. Due to your head trauma, you also have hallucinations all through the hospital and other levels later on where mutant cats and insanely annoying Gary Coleman demons pop out of nowhere and you have to fight your way through them to the point where everything goes back to normal. When you leave the hospital, you get hungry and decide to go to the Chinese retaurant a block or so away. When you make your order, all of a sudden a bunch of mad cow diseased zombies with tourettes start popping up and you have to go through the restaurant slaughtering these zombies. After that you are sent out on a bunch of missions for Vince, the guy who fired you in Postal 2. Basically you do tasks like that for about three more hours and it all ends with one of the funniest boss fights that I've ever played, but you'll have to get there yourself to find out why.
All of our favorite enemies return for Apocalypse Weekend, and they are still basically the same as before just in new areas and there are a few new skins for them. This may not sound like much, and in reality, it isn't at all, it is a lot better than Postal 2. Instead of carrying out boring errands for your wife, you're now actually doing something worth while (fighting hordes of zombies, killing countless amounts of Al Qaeda, breaking out of a prison guarded by the army, blah blah blah).
The first thing I noticed was that the level design had improved a lot from Postal 2; there were a lot more props and the textures were much sharper than before. The models also got a little better, but not too much better. The weapon detail has not changed much, if at all, but the new available weapons definitely look better than the old ones.
The ragdolls are sadly just as bad as they were before. Corpses throughout the level will get stuck in walls, in the ground, on the doors and sometimes they start shaking because of it and cover your screen with ridiculous amounts of dust, causing your FPS to fall dramatically. The only satisfaction you get from that is that the corpse explodes into a bunch of bloody little bits after a few seconds.
The weapon arsenal in Apocalypse Weekend is much better than it was before. With the exception of the ability to pee on people, you still have all of your old weapons and a few new ones. There's the sledge hammer that automatically decapitates whoever you hit with it, there's a machete that can cut people into little bits, and there's the scythe which can cut people in half and into little bits as well. There are a few other new weapons, but I'll let you find them yourselves.
There are 20 or so new areas for you to travel through, but you cannot revisit any of the ones from Postal 2. Also, since Apocalypse Weekend follows a more strict story, you don't really have the freedom to go where you want anymore, which is both good and bad. It's good because instead of just slapping you into a boring world with just about nothing to do, it now throws you a few curves and keeps you interested if you haven't thrown up about twenty minutes into the game and it's bad because you don't really have any freedom at all anymore.
Apocalypse Weekend is much more disgusting and vulgar than Postal 2 was, and it only makes things funnier. Since you can now cut people's limbs off, things are much bloodier in that respect and since Apocalypse Weekend is much more story driven, the dialogue is only more hilarious. I don't suggest that you buy this game if you are squeemish or do not like a lot of meaningless foul language because this game has tons of blood and a ton of language, so it's definitely not something that your parents want to see or hear you playing more than likely.
Overall I'd have to say that this game is worth the buy if you have Postal 2; if you don't have Postal 2, then you can't play it and Postal 2 is really not worth the money.