The game is definitely more fun with others. However, like everyone has said, the game is still playable and beatable, as it was meant for single play. Connectivity was an added bonus. Other players could join your game, via local wireless, and vice versa. But what everyone else did NOT mention was Map Trading, as you could've traded treasure maps with other players. They had no connection with the story whatsoever, but they were a fun thing to do on the side.
i actually had a couple friend who got the game as well and when he hung out one time, I trolled them by having them join my game and fought Lv.99 Dracolord (whom I've previously beaten) and we all got wrecked. As consolation, I gave them the Masayuki Map and ran them through it a couple times. My only regret is that I never got Kawasaki's Locker.