Fun treasure hunting and interesting boss fights surrounded by a weak story and questionable gameplay.
You do gain more abilities and whatnot throughout the game as you move through a variety of levels, but the meat of the game is fairly repetitive. Fighting enemies is fun, but get's repetitive as similar enemies haunt every stage and you'll find yourself using the same strategies over and over again.
On the other hand, the boss fights are fun and original, but none of them are particularly hard. Most of them can be beaten in your first try, since continuing when you die takes you right back into the action by spending a hunk of collected coins, which is an interesting alternative to collecting lives, but adds to the game's ease in the way it's done. Even the final boss is disappointingly easy, and can probably be beaten without even continuing on your first try.
The best part of the game is the treasure hunting. Going around collecting everything that there is to be found is the only real way to really get into the game. Of course, there's little replay value in this, because once you collect everything, there's really nothing in the game to do.
The most annoying part of the game are the cliffs. In old school games like Super Mario Bros or Mega Man, falling down a pit sends a character to their doom and you have to begin the level again at the last checkpoint, or the beginning. In this game, falling down a pit brings you to an underworld type room with platforms, acid, and ghosts that try and knock you into the acid. Doing so loses you coins. You have to run around breaking boxes to find a spring to launch yourself back up to the place where you fell off. These subworlds do increase in difficulty as you progress to higher levels, with less place to stand and more acid, but they fail to be very interesting, and get pretty annoying, as one tends to fall into them a lot while searching the nooks and crannies of the levels to find the treasure.
The graphics aren't bad, but there's nothing that separates them from that of any other game. I would equate them to Sonic Adventure DX: Director's Cut, but with better animation, and no awkward mouth syncing. Speaking of the voices, the sound, too, is pretty much what you'd expect. So these aspects of the game don't hurt it, but do nothing to go above and beyond.
So, if you want a game you can finish quickly with lots of stuff to search for, you might want to give this a rent. Personally, I found the treasure hunting addictive, but without that aspect, there's really nothing to this game.