GMR is an easy to pick up game, but has a lot of tactical options. There's a lot of flexibility in creating your own monsters, so you can use whatever monster figures you have laying around. Overall, there's a good mix of ranged and close combat options for the monsters along with special abilities that enhance attacks/movement/etc.

The only issue I have with GMR is that it would be nice to have a couple sample monsters available in the core rulebook.