This game is a blast. I love jumping off of walls and trying to beat my own time. TONS of levels included, free DLC (and some to buy) adds many many hours of fun for a pick up and play adventure. Some levels are very challenging and take a few play throughs, but still keep your attention and desire to win. Also comes with a level creator to make fun new levels to challenge your friends. Though I marked it down to a 9 because there is no way to share this content as there is in Halo 3. I have created some great maps but really are just for challenging myself.
I created a map asking my girlfriend to marry me so it was definitely worth the purchase price to me.
If you are unsure, give the demo a try as it is a descent representation of gameplay...but not difficulty.