It has a very confusing user interface. your character starts off at level one. By the time I reached level 15, I realized how to use the interface to my advantage. I LOVE THE GAME, but it does have some shitty elements. The user interface is very complex, but worth playing just to learn how to use properly. The lack of a free roam camera does occasionally mean that yyou die before you successfully turn around to kill the guys behind you.
You have five different character classes that are all completely different and your characters behave, level up, acquire weapons & abilities very differently.
It's worth renting to play for the sheer volume of deaths you can cause with a grenade launcher, and the air melee combo's you can rack up if you apply your hard earned ability points to the right abilities.