Perhaps the greatest feature in this game is that most weapons are fully customizable. Everything from the accuracy, power, fire spread, particle velocity, number of particles, napalm spread, everything. Want a mini gun that fires 200 bullets in a straight line? Got it. Want unlimited ninja ropes? Got it. Want a napalm bomb that spreads fast and lasts for rounds? Got it. The future games just don't let you tinker with these numbers, and that can take a lot of fun situations out of the game. These aren't missed in the new series, and there are other things that they add to make up for it (such as low gravity, etc), but the Potential for fully customized weapons makes this a completely rocking game.
Other features include a full map editor with a random generator that makes decent cavern and open maps. You can also add "borders" to the game world, so that the worms never sink. This can be great fun on a cavern map, as you ninja rope your way around. Or, go for an open map with a bottom border to allow airstrikes and the killer mortar strike (stand on an opposing worm's head and fire a mortar straight up. It'll hit the worm on the head, exploding all fragments at once. Usually, instant kill).
As for the "campaign," I suppose it's decent. I really can't recall it as I usually just skirmished.
To date, I've never gotten this game working on Windows XP, though I'm not sure if that's my copy, as the game is quite old now. Just a word of warning.