The Demo was really short, I think I beat it around 10 minutes or so. I would give it about an 7.5/10 for the demo. Might be a lot more fun in the actual game because of the items you unlock later.
I like it because it's kind of an stealth game, if you aren't patient, then you might have a hard time beating the game as they use guns and kill you before you kill 1. But when you aren't doing the stealth part, it's just another beat' em up, tends to get repetitive too.
I would recommend getting the demo first if you actually plan on buying it.