Don't get me wrong, they're totally addicting. That's the whole point. Each stage opens up a new thing that makes you a little bit more powerful so you keep thinking "now I can do This", but each stage of the game takes longer and longer and ultimately, nothing really changes. You're still do the same thing over and over again. It's addicting, but I get no joy in the experience. It's just hard to stop.
Meanwhile, the non-optional PvP element is basically just stealing the hard-earned resources of other players. You log into the game to find another player has stolen resources it took you hours or even days to gather. Sure, you can attack them back, but them you lose the temporary protection obtained by having your base destroyed. And what's the point? Now they've lost time and resources.
This experience could be totally turned around by adding some additional strategy or RTS elements. As is, your attack units are completely mindless. After dropping them on the battlefield, you have no control over their behavior. They'll attack random (and strategically worthless) walls while being fired upon (and destroyed) by enemy units. Your defenses seem woefully underpowered (at least that's my experience) at repelling invading players. A single enemy unit was capable of taking down both shield generators and three turrets before being reinforced by the rest of the army for wiping out the rest of the base.
OK, rant over. And so is my time with this game.