I own PSO for the gamecube, so I\'ll try to answer your questions.
Yes, you get it online by buying either a GC broadband or modem adaptor.
Online, the game is great. It is really one of the better titles for the Cube if you play it online. However, you have to play it online to appreciate it. Offline, it really isn\'t all that great. It gets a lot more repetitive and dull. While it can be played offline, and is decent if you play it with others, offline, I would really recommend playing it online if you\'re going to get it. Offline, it isn\'t anything special at all. Online, it is.
So basically, if you feel like paying the monthly fee Sega is charging to play it online, then get it. If not, then you could probably find better games to spend your time and money on.