It's such a fun game and the base mechanics are really fun. The combo's you can create with upgrades are quite diverse which adds to the replayability.
The only gripe of mine was that by level 20 It's like 70 or so enemies to defeat and it just takes ages. Perhaps some boss encounters or something to mix it up would add a bit of change :D
Still, a great fun game that definitely sucked me in for 1-2 hours! Well done :D