Opening caveat: I am not traditionally a person that plays Tower Defense games. This is in fact my first one.
To get right to the point of it...I loved the game. If the name wasn't a dead giveaway, the game has a silly, lighthearted tone to most of it that, when combined with the creative ways you can inflict mass orc genocide (some favorites being hurling them by the packs into bottomless chasms with a well timed gust of wind and launching them ragdoll-style for their afternoon lava bath), leads to plenty of destructive catharsis.
While you can play all the missions single player...the game is at its best (IMO) when played with a friend. Many a map benefits heavily from having a second mind on hand to help combat the games challenges, and with co-op your individual action bar is halved so it emphasizes working and planning together.
The missions also reward player skill with the main currency of the game, skulls. Doing things like beating a mission within a certain time limit...not letting anyone through your defenses...and even "hidden" bonuses like not using mana or not taking damage all reward you with additional skulls after the level. If however you're not that great at pulling things off that way, levels can be farmed repeatedly for unlimited skulls so you aren't punished harshly.
In conclusion...recommended. Get a friend...get some chips...and get killing.