They're probably my least favorite games in the series. With the exception of Mario Party: Island Tour, I like the Mario Party games made by ND Cube the most (10 and Star Rush are my favorites).
Even the N64 Mario Party games are more fun because of what you can do with Gameshark codes. I use a Dice Roll modifier so that my character rolls a 7 every turn and that way I don't have to worry about getting crappy low numbers. I also use Character Modifiers to change my opponents mid-game. What I like to do in Mario Party 3 is start the party with characters that I like (I play as Peach, my opponents are Daisy, Yoshi, and Donkey Kong), and then on the last turn I change my opponents to Wario, Waluigi, and Luigi so that they're the ones that lose at the end. I can't do this in the Gamecube Mario Party games because I haven't been able to find any Character Modifier codes, and that's why they're not as fun to me.
There are other problems besides that, for example, I really don't like the board gameplay in Mario Party 4 and 5. Both games are painfully slow since the characters move across the board at a snail's pace, and some of the items have really long animations. Also, all of Mario Party 4's boards have some annoying gimmick that makes it difficult to get to where you need to (teacups in Toad's Midway Madness, the roulette and Goomba dice block duels in Gooma's Greedy Gala, the banana peels in Koopa's Seaside Soiree, etc.).
Mario Party 6 and 7 have much better boards and the gameplay has been sped up, but their minigames are mediocre. Both games have a few minigames that I really can't stand playing and make me want to reset the game whenever they get chosen (Cannonball Fun and Same is Lame in MP6, Bubble Brawl and Sphere Factor in MP7). Mario Party 7 also has one of the laziest and least creative minigames ever, which is Cointagious.
So yeah, sorry to the fans of these games but I really don't like them anymore. I said that they're probably my least favorite games in the series, but I think that I actually dislike Mario Party: Island Tour more.