After playing for a few hours, I can say it's pretty fun, although it feels like a flash game.
It's not bad, it's actually pretty good, but it's definitely not a complicated game. You have some marines, who are good at shooting things. Their job is to clear the area of alien hives, which they do by killing the hordes of creeps it spawns and slowly making their way toward the hive so they can kill it. You have BP, which you get for clearing hives, which you can use for buying weapons, buildings or more marines (or marine capacity, since they respawn after they die). The basics are simple, but it can get pretty hard. After a few levels, the hives start spawning turrets, which poison your marines and kill them, and at roughly the same time you unlock healers (unless you're an idiot and buy the one level boosts instead), which outheal the turrets. The maps then get multiple pathways and more hives, which means you now have to balance using BP to help your marines take bases, as well as to protect your bases with turrets. Then, once you get comfortable, the hives start getting mutations, which do things like spawn mines, heal buildings and make spawn rates increase when a hive is captured.
They definitely nailed the pacing. So far, I haven't felt out of depth, but I haven't gotten too complacent either. I haven't lost, so far, but I've gotten quite a few nasty surprises. There might not be much complexity to the game (might get more complex later on), but what it does it definitely does well.
I'd say if you don't mind games that are lacking in mechanics, as long as those mechanics it has are good, this game might be for you. If you like games like Creeper World, where you have to balance your resources and slowly make your way to your objective, then this game is probably for you. Personally, I really like the game.
Does it have the same barely controlled chaos feel that things like harvest and creeper world give you?
Pretty much. You have to build turrets to prevent the creeps from taking your bases while you push for another, which means you have less BP for your marines, which means that they might get horribly murdered, and you lose your bases while they respawn. The first levels are easy, but they get gradually harder and more uncontrollable. I'd say it's a bit easier, since your resources are more of a unit cap than something you lose, but I haven't gotten that far yet.
Edit: About what I said about it not being complicated early, you definitely get more options later on.