It's really fun, but once you get the hang of it, it get's easy. I reached level 92, but I dare say it's possible to get even higher. Just try to use the different colors as effectively as possible... good for the front towers f.e. are red and lime gems, as they can hit multiple monsters.
I have gotten to level 84 without replaying any levels more than twice. The author pushed back the release date for GC-2 for an unknown date in January. I can't wait. The secret is using red and lime gems whenever possible! that and to send as many lines as you can defend against at a time, to up your score.