I'll modify my statement now that I've played most of the day. It's addictive, but it's also getting frustrating. The missions are getting harder, and I can't seem to get ahead of the curve. If I was playing co-op, it would probably help. I also keep spending too much money replenishing my ammo and health packs, and the really good items cost a ton. Also, I spent a lot of skill points upgrading my turret's firing power, but even maxed out it's not all that effective anymore. I suppose it does still distract some of the enemies when there are a ton of them, but it would be nice if it was dealing greater damage. I can pay to reset and reassign all of my skill points, but that costs a lot, too. Respawning also costs money, so yeah, there are always things keeping me from getting the best items available.
I don't really know where the story is going. So far it's just a bunch of missions with no discernible connection, but there's a Cortana-like female presence who communicates every once in a while, so I know it's going somewhere.
My character is only up to level 16. My nephew is at level 32. He and my brother-in-law have been playing it a lot, I guess, and it sounds like they're getting as frustrated as I am. Here's hoping we find ways to make it easier.
Make sure you accept and complete every mission, especially in the begining of the game. If you don't, you won't level up enough and your character will be underpowered once you get further into the game.
One other thing to note: the mission difficulty levels are "Trivial, Easy, Normal, Tough, Hard, Impossible". Don't make the same mistake I made. I though missions rated "Hard" were easier than missions rated "Tough", and I couldn't figure out for the life of me why I was constantly getting killed on a certain mission.
And yes, playing the game with friends helps. I've been playing with two of my friends and while the game does get difficult at times, it hasn't been frustrating (besides for the one time when I took on the "Hard" mission instead of the "Tough" mission).