OMD was my first venture into this genre and I really liked it. I also picked up Dungeon Defenders on one of the crazy Steam sales, and somehow sunk over a hundred hours into that! Is Dungeon Defenders well liked around here? I don't recall seeing much talk about it.
It came out at a great time because I was itching for some action-RPG like loot, and Dungeon Defenders has the tower-defense/shooter combo and loot. I also really liked DD because three other folks at work were playing it along with me, and there is some great multi-player class synergy.