I had a good few throwaway hours on it, just run around and kill stuff. Some plot came along (I think) which made me better at killing stuff and then made the stuff to kill harder. Repeat.
I spent probably 10-20 hours on it, had some fun, then got near the end to a bit I kept dying on, got bored and haven't touched it for the past 6 months since.
It's not a good game by any stretch IMHO, but if you're after a fun but repetitive button basher that will kill a few hours, I'd say it's worth a fiver personally but am well aware a lot of people will disagree.