I picked it up when it went on sale last week and burned through it within 24 hours. I don’t even remember the last time I couldn’t put a game down like this.
I didn’t find the difficulty too bad, and I thought it actually got easier as the game went on. Once I got the jet pack, I was able to maneuver around the map much easier, and considerably more so after upgrading it. After a bunch of defense upgrades (which are fairly cheap), taking damage seemed almost trivial. The final boss did take me a couple of tries.