Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
As you have probably read from official review, the initial release was terrible. However the original is now been replaced and has been patched to correct things; such as now people ragdoll instead of falling straight down, there are dozens of difficulties, featuring Libermode, a poke at Senator Liberman who was very angered over the original Postal, and Hestonmode, named after the actor and leader of the NRA Charleston Heston who mentioned several times throughout the game.
Sharing the Pain: Postal 2: Share the Pain adds the exciting realm of Postal Multiplayer. Which in contains the same modes as every other multiplayer game, Deathmatch, Teams, CTF (two versions, stealing money or the postal babe), and several others.
Apocalypse Weekend Expansion Pack has now been released also and adds two more "levels" with a "story". It also adds 11 multi-player maps, level/scenario editor, more weapons including the scythe, boomerang machete, and sledgehammer. Attack Cats, Gimp Dogs, zombies, and more Gary Coleman are included.
With it's corrections and multi-player the game has changed a great deal. Although it still has little replay value. Multi-player is fun, if you can find an empty server in the short list.