This game is hands-down the most fun to play with my friends, and has always been since I first bought it.
Anyways, it's safe to say I fell in love. I played very much on Single-Player, but most of that was for the reason of getting other maps for Multi-Player: where all the fun begins. Me and all my friends would always play, and I would always try to keep their characters not too far behind. We would just sit around and play for hours on end. This game is very addicting, and, though not FAST-PACED, it's more action than most RPGs, and very very fun.
The game has a great ambience to it, with the music, and colorful 'city' within the Pioneer. The game is just.. Amazing, honestly. Especially when first released on Dreamcast, and having amazing graphics (for its time). I love this game, and miss it.
Of course, I began to play Private Servers for PSO *gasp*! Z0mg, Private Servers are illegal! But the thing is, the PSO:BB offical servers are completely offline. It led to even more fun, finally being able to play offline, rather than playing in split-screen.
The only real reason this game doesn't achieve higher is the repetitive single-player. But that's not to say it is terrible, it's still well worth playing, and who knows. Maybe this recommendation may make YOU fall in love with it.