The singleplayer is actually nothing special, as it's just a series of missions where you blow up enemy tanks around the world. It's hard to expect much more, or much less from a game like BattleTanx. There's not much to unlock but there's at least a bit of fun to be had if you don't have anything better to play.
The multiplayer is what this game is all about. The variety of tanks, large number of maps, huge variety of special weapons and the various factions all make for an extremely hectic and fun battle experience. With 4 people on the splitscreen, this is definetly one of the most fun multiplayer games I have ever played on the N64. Really, it's only second to the likes of Super Smash Bros, Mario Kart 64, and the like. There are hover tanks, giant Goliath tanks, laser tanks, and even... yes... the MotoTank (a cross between a motorcycle and a tank, for the uninformed). It's all over the top destructive craziness with swarm missiles, mines, guided rockets, lasers, plasma, and even nuclear weapons (even in multiplayer). These are but a sampling of the different weapons. This, combined with the fact that you can destroy buildings while you fight, is really impressive for the N64.
The graphics are quite well done, with weapon effects and tanks looking really outstanding. There is slowdown, sometimes, with enough tanks/weapons firing on the screen. This is one of the better looking N64 games for its time. The sound is decent, but nothing special. For a simple, fun multiplayer game BattleTanx: Global assault is really good.