Levels 20-29 are terrible as you are just replaying content from levels 1 to 19 albeit at a harder difficulty, hoping that will be fixed with this weeks update, it is only about 3/4 of a game at the moment after all. Levels 30-40 have 3 new levels to experience which really ramp up the difficulty.

It's going to be Free To Play on Vita too btw and they're hoping to make it so Vita players can play with PC players. It's also coming to smart phones later in some form using your PC save data.