Whoever said over-rated by N-fans, underrated by the rest is probably right. For a Nintendo fan, there's shit-loads to own.
In terms of third party content it's not so great for western style games, but it's great for quirky japanese games and arcade oriented multiplayer experiences.
Those who haven't even tried Gamecube and it's most lauded titles are doing themselves a disservice. Try before you judge.
I own:
1080 Avalanche
2002 FIFA World Cup w/ Football Mat
Baten Kaitos: Eternal Wings and the Lost Ocean
Beyond Good & Evil
Donkey Konga
Donkey Konga 2
F Zero GX
FIFA Soccer 2005
Freeloader - multi region disc
Gameboy Player
James Bond 007: Everything or Nothing
Killer 7
Luigi's Mansion
Mario Kart: Double Dash
Metal Gear Solid: The Twin Snakes
Metroid Prime
Metroid Prime 2: Echoes
Need for Speed Underground
Need for Speed Underground 2
Phantasy Star Online: Episode I & II
Pikmin
Prince of Persia: The Sands of Time
R: Racing Evolution with Pacman VS
Resident Evil
Resident Evil 0
Resident Evil 4
Soul Calibur II
Spiderman 2
Star Fox Adventures
Super Bust A Move All Stars
Super Mario Sunshine
Super Monkey Ball
Super Monkey Ball 2
Super Smash Bros Melee
Tales of Symphonia
The Legend of Zelda: Collector's Edition (Zelda I, II, OoT, MM, WW demo)
The Legend of Zelda: Four Swords Adventures
The Legend of Zelda: The Wind Waker
Timesplitters 2
Viewtiful Joe
Wario Ware Inc: Mega Party Game$
39 games. About 12 of which I could have bought first on other systems. A couple of those were out on Gamecube first. I like a good majority of these games... the few bad eggs aren't worth anything in terms of trade in value and thats why I still have them.
This is compared to what I now own on PS2 and Xbox after selling stuff I don't play anymore: 7 PS2, 15 Xbox. It all depends on what you like. For a lot of people PS2 is obviously enough. Other people still require an Xbox or Gamecube - either for playing on their own or to suppliment their PS2. That just seems to be the way it is.
I personally feel the GC is underrated and missed out on by many, but I do think some part of it is down to personal preference and other systems having better versions of third party games (ie. online only on Xbox, better graphics on Xbox) or simply more of them (ie. PS2)