The characters are pretty fun, the story is far fetched but I have played many games with worse- for a shooter it is pretty engaging.
The weapons are unique and have fun effects to them, each character you can play as has different strengths and styles, so you can mix things up and keep things fresh.
I did find the combat to get repetitive in parts (enter a room, a horde of enemies flood in and take position. Clear the room, go to the next, repeat). there are a number of boss fights that are fairly unusual for a shooter, and each requires a different tactic and presents a different challenge.
Long enough without outstaying its welcome.