The environments are very well designed. The human animations created a very atmospheric experience. The concept was well thought out and coherent. The included tutorial, a text document, was very comprehensive.
The battles were exciting at the beginning. I wish multiple rats could fight simultaneously, perhaps even in teams. I'd even suggest that you don't necessarily need your own rat; you could simply choose one from the fighters currently engaged and bet on it. Maybe even use items as wagers.
I enjoyed it. I leveled my rat up to 14 and bought another one with a fast attack. It was worth it :)