I liked the graphics, and the concept was really good, but I think the arena is just too large, and the wave timer too short - It's too easy to get a massive group of enemies you can't shoot through because enough of the enemies in front were of the wrong type because it takes too long to walk over to the switching areas - When there wasn't massive groups of enemies, the game was super fun!
Overall still a really good game, just too easy to get into an unwinnable state I think.