Really fun shooty-bang-bang game.
There is loot and collectibles everywhere. You're characters have levels which increase with progression, from what I've seen each level gives you a permanent 2% damage boost. There are a couple different currencies in the game which you collect and use to upgrade your characters and unlock abilities. Enemies have health bars which the game says have different resistances to different weapons, but on the 2nd hardest difficulty, at least early on, the sturmweger and shotgun seem to be strong enough to quickly take down anyone, except for the huge robot dogs which require a laser turret.
I'm playing solo and the AI partner is actually pretty good at distracting enemies, and has revived me more than I've revived her! It never gets in the way and when you're crouching it just turns invisible.
The game takes place in an underground hub and you just fast travel to different areas to carry out missions. Even in the first level, there are rooms that require a specific ability to open that you couldn't possibly unlock early on, so obviously the game wasn't designed for linear progression, but rather an arcade approach.
I'm having an awesome time with it and I'm very interested to see this story through.