Overall, simple and effective. The game's mechanics are easily understood, while the effort put into the bosses keeps it from becoming boring too quickly. The implementation of the theme, if basic, is amusing in its own right - the fact that combat revolves around throwing cinder blocks at sheep (and later, eldritch monstrosities) gives the game a charm it wouldn't have otherwise.
The only real issues I noticed were from the final boss - the boss itself is very spongy relative to the variety of its attack patterns, and it's hard to tell where its laser attacks will hit at times.