There's been three installments in the Mario Party series to date, and I've played and enjoyed them all. In brief, Mario Party puts you and your friends in control of various Nintendo mascots on a map which looks and acts just like a board game. You roll a digital die and move your spaces, choosing a path when there's a fork in the road, attempting to sabotage your opponents with items you've picked up along the way, and gathering coins and stars by landing on the right square at the right time. If that sounds boring, don't worry, because that's only half the game. After every round, a small game takes place -- sometimes it's a matter of hitting a combination of buttons as much as possible, sometimes it's memorizing a pattern, sometimes it's a matter of precision, still other times it's a matter of russian roulette.
Yes, it's a simple formula. Yes, it's possible to play just the mini-games without futzing around with the board game. Yes, it's a game that isn't fun in and of itself, but which is fun because you're playing it with your friends. If you regularly play four-player games, and you're actually getting tired of Smash Brothers or Tetris, etc, then this is a perfect addition to your game library, and is well worth the money. If you're solely a solo-gamer, then you can spend your money on something better suited to your unique habits.
The graphics in all three versions of Mario Party are about the same, as there's only so much that the poor little N64 is capable of. There are subtle enhancements as the series progresses, but they're hardly worth noting. Luckilly, Mario Party is also a game which doesn't need stunning graphics. It's only a framework to having a good time with some friends.
The sound is good throughout, with lots of catchy little musical tunes. Notably, the intro theme from Mario Party 2 frequently pops into my head and brings a smile to my face. On the other hand, the character voices are used a little too often (though it is not as bad as the insanity in Super Mario Advance), and if I hear Toad shout Ya-Hoo! one more time, I'm going to go nuts. But on the whole, the sound and music doesn't get in the way of the fun.
As hard as it is to explain, the theme went from very lighthearted and whimsical in 1 and 2 to dark in Mario Party 3. It's not a bad change, it's just an unusual change. I think the main difference is the music is a little darker, which leaves a subconscious residue coloring my view of the rest of the game. I don't mind the darker mood, but while playing Mario Party 3, I did find myself craving the lighthearted fun of Mario Party 2 from time to time.
As far as the mini-games go, they come in all shapes and sizes, and between all three Mario Party games, well over 100 little games have been introduced. Most are good, but as with any sample that large, there are a few rotten ones. Every time "The beat goes on" came up, for example, we would all let ourselves lose, just because the game is so annoying. And each of us has mini-games that we excel at, and games that we can't do well at for any reason.
If you're only going to play one of the games, I would recommend 3, because it had a lot of enhancements added. It has more mini-games, more modes of play, and more variety in the board game element. If you were going to try out two, I would suggest 2 and 3. It's not worth your time to try all three at this point, though.
Or, instead of trying any of the N64 installments, you can wait until Mario Party 4, which is due out on the Gamecube some time next year. What I would like to see in a Mario Party 4 is a return of the lighthearted feel of 1 and 2, with the systems of 3, and all of the mini-games from the first three games (yes, even the bad ones, because bad is subjective) as well as a ton of new games. With the new Gamecube mini-dvd format, a single disc will be able to hold all of those games and more.
If you enjoy playing video games with a lot of your friends, then one of these three games is for you, and at the very least you should give one a rent. Any one of them earns a 4/5 on it's own merits.