Since the first game was released 1999 Nintendo have been "doing an EA" with the Mario Party series - and we're now on the 5th version in the space of 4 years!
Mario Party 4, the first gamecube version - and the first version that I ever played - was at the very least, extremely good fun with 3 friends. A total demonstration of what makes nintendo great. Now I'm not saying they should release a new one every few months - but it is clearly a great series, really brings a smile to your face. The GameCube has some really great multiplayer games, and IMO at the center of it is the mario party series.
I don't think your expected to buy every copy - maybe if you buy every-other version then you'll get more out of it. For instance, if you own MP4 then theres little point in getting MP5 - but if you haven't got MP4 and maybe you own and love, say, MP3 on N64 - then MP5 is deffinatly worth it. Of course, if you haven't got a Mario Party game before - then what are you waiting for - go get!