For those with lots of time and patience.
Not the best of graphics, but then again it's for GBC, so what do you expect? And I've definitely seen much worse compared with this.
For gameplay, there are many choices regarding party members, and it's quite fun to try out the different combinations. You can always go solo, and that gives it the extra bit of challenge (already plenty in the game) that you desire.
This game is also quite long, with most of the time spent leveling up, but most of the time I didn't mind it at all.
To sum everything up, this is definitely the best RPG for GBC.