I agree -- mechanically, the bosses are all pretty uninteresting, although I find most to be pretty damn rad looking. Playing the game over the years, I always kind of feel like I either outgrinded (outground?) the boss through XP and gear, had enough healing items, or accidentally won (I am by no means the best at this game, though). As action game bosses, I feel like they're pretty wonky.
But yeah, still one of my all time favorite games :]