Overall, the visuals are very good. The controls work well, though it's going to take me a long time to master them. I still hit the wrong button sometimes. If you get caught on foot you die real fast. The weapons are pretty fun. The mission structure is very much the same as Borderlands. Haven't done many challenges, but I've done enough story missions and side missions to know the game is solid.
I tried multiplayer and it was really difficult. You need to get huge scores to get rewards.
I like that the collection shit actually does something. They're materials to make amps. Then there's a defence game to actually make the amp. It's pretty good.
There world is pretty big, but I don't think it won't have the same sandbox style play as GTA, because you don't have enough interactive elements.