Was one of my first vita games, and is now one of my favourite slashers ever ;) Great gameplay, easy to learn hard to master. Level design leaves a little to be desired in places (looking a little samey round the city levels), but has some excellent locals (such as hell :P ). Definitely worth picking up - especially if you're a fan of DMC or GOW and can get it relatively cheap ;)
I played the crap out of Ninja Gaiden on the original Xbox and loved it to death. I bought it on Vita a couple months ago and only put an hour or so into it. I just can't get into it on handheld. The controls are cool. I don't like the back touchpad ninpo controls but everything else is fine. If you light challenging action games then this game would be great for you.
It's the worst version of that particular Ninja Gaiden game. 30 fps and Vita's analog sticks do not lend themselves well to games that require precise movements. Ninja Gaiden Black > Ninja Gaiden Sigma > Ninja Gaiden > Ninja Gaiden Sigma Plus