Each dungeon you play in starts off pretty slow, but usually by the second or third wave their are enough monsters to make the game entertaining. The gameplay can get repetitive after a while, but only after playing for many hours in a row, however at least in my case after taking a couple hour break I am able to put more time into it. The constant gear upgrades and level ups keeps you wanting to continue playing.
For what your doing, the graphics are just fine. They are kind of like Borderlands graphics, with everything having a black border.
Dungeon Defenders also includes an online function. It performs exactly like Single Player, but with more monsters based on the number of people that are playing with you. It may sound simple, but playing with 3 other friends against one of the major bosses is fun that simply can't be found in other Multiplayer games.
Final Score: 89
This game is well worth the $14.99 price on steam, and will give you hours of entertainment both in Single Player and Multiplayer.
Steam Link: http://store.steampowered.com/app/65800/