I finished my first playthrough earlier today (with the Amazon). Overall, I liked the game a lot more than I thought I would. I thought about cancelling my order a few times due to lack of interest, but I decided to keep it because of magus' enthusiasm. I'm glad I did.
The game is a little too heavy on the RPG elements for my liking, though. It would've been great if they managed to keep everything balanced between all of the loot, leveling, and skill trees, but it looks like high level characters can tear right through Inferno mode if their gear is strong enough. Or alternatively, they could've made an arcade mode that has predefined character strength, skills, equipment, and a linear series of stages - sort of like Hard Corps: Uprising. I think that would've been the best solution. I guess it's not that big of a deal though since the game's pretty fun as it is.
I also tried playing with some allies, but I kept losing track of my character and every battle just seemed like a big clusterfuck, so I went through most of the game solo.