Not sure if it is for everyone, though for me I am quite happy with the purchase. I'd say wait for the demo and that should tell you right away if you should buy it.
PSM said it could be finished in 10hrs, but they also said that was just doing the bare minimum to finish everything in a linear way. There is a LOT more to the game than that since you can wander around and talk to other people (NPCs)...there are little "mini" challenges, some related to the story and others not. As you wander and talk to people, you find "named" characters - people who either give you information depending on where in the game you are, or ask you for help and they give you info on how to complete your next main task or unlock content for completing their "quest". For one task, an old lady lost her bracelet on the subway tracks and you have to get it for her without getting splattered...stuff like that, that gives it another interesting dimension. You don't HAVE to complete all the tasks that aren't related to the main story or talk to these people, but it makes it fun IMO.
There are also many (at least 100 as far as I can tell) collectables that unlock content and some require a little bit of thinking to acquire (like using various powers or summoned helpers), others you just have to explore around for.
This game earns its M rating without even breaking a sweat...
I really enjoyed Sigma

I had played the demo enough that I already knew the strengths and weaknesses of the weapons so I wasn't coming into the game without experience....though you basically only have one type of enemy in the demo and certain weapons aren't good against all enemies, or groups of enemies. I finished the demo on hard difficulty, but couldn't get half way on "very hard". I did beat the whole game with an overall ranking of Head Ninja, because I backtracked on a few levels because there are some things to explore there also, so I got low rankings on a few levels (Ninja Dog, LOL). Some of the new weapons are pretty cool too...I like the flail.