NOTE: As the previous writeup was made from a Dreamcast perspective (PC Worms have had online play since Worms2), this will be from a PC perspective.
The fourth Worms game (Not counting the Reinforcements pack for the original Worms, or Worms: Director's Cut for the Amiga). Worms World Party is nearly the same as Armageddon. There aren't even any new weapons this time around.
However, there are a few new options. The Wormpot modes add all sorts of odd features to the game, from simple things like giving certain weapon types (unarmed, handguns, explosives, etc) more kick or more damage (Further blasts or more pain), to making all crates contain sheep-based weapons, to the slightly more drastic changes, including setting up two forts from which the teams have to attack from, setting up options where you only get weapons that you 'buy' (You get 100 points, each weapon costs X amount), and, of course, earthquakes every turn.
And, of course, there's the missions.. not much new if you've beaten W:A's missions (Other than 40 new ones), but this time there's multiplayer co-op missions. Plus, the Mission Editor allows you to make whatever sort of missions you want.
I haven't had the chance to try WormNet with WWP, but it still seems they don't have rankings up and running again (They had to take them down and resort to unranked players after some bastard cracked the server and was able to log in as anybody)... plus, it's still a bugger to run it behind a firewall that isn't SOCKS-based.
All in all, if you found Worms: Armageddon fun, you might like the extra options in Worms World Party. If you didn't, you won't find anything new. If you're just a wee bit obsessed with the Worms series like I am, go for it.