For single-player, all you need is the Gamecube controller. If you want, you can connect an additional GBA screen for the Moogle's "radar" I think, but I haven't tried this yet.
For multiplayer, every player needs their own GBA/GBA-GCN link cable. Don't complain about it here, the GBA is required to have the full experience. Sure it may be inconvenient but it's better than having menus clog up the whole screen I think. The primary purpose of the GBA is so everyone can use their own menu screens instead of pausing the game every two seconds to switch items.
Whether you think it's worth it for just the single-player is your decision. I haven't tried the multiplayer yet but I heard it was amazing. If you're into the mainstream FF's and is expecting a mainstream FF then you will be disappointed. Likewise, if you're not into FF then you might consider a rental.
That said, I may comment that the graphics are simply amazing. In fact, some of the enemies are too animated to the point of being obnoxious. Moogle's are the cutest they have ever been. The music sticks to a medieval theme, but remains good, yet repetetive like most FF's.
On a last note, other than the moogles and pheonix downs and small things like that, it is very much unlike the "regular" Final Fantasies. No leveling, heart-style HP, no MP, and it's all in real-time. But I still consider it as much of a FF as FFTA was, because I read once that the whole FF series is not supposed to be similar, and that one should be able to pick up each game without having to compare it to the previous ones.
I didn't get all that far into the game yet, so I'll say more about the actual gameplay later.