A lot of fun. Don't be discouraged by the semi-slow start, because it gets much, much better. It looks gorgeous in its simplicity, it's addicting with all its unlockables and randomized cube behavior, and just keeps giving players more.
The game remains a lot of fun and stockpiling a bunch of alternative spheres and unleashing them in one epic battle can be a blast. GameCenter integration provides global leaderboards and 3 dozen achievements to earn.
While you may be looking for the next high-definition shooter for blasting zombies or aliens, it's worth taking a side-step to play Cubes vs. Spheres as it combines simple action based fun with a unique setting to keep things novel.