I picked up N+ for the DS on a Black Friday sale for like, $3.50 or something equally ridiculous...but forgot to play it until today. Review at a glance? Maybe 6-7/10.
It's a fun game, but oh my goodness...the music. It is horrendous. Do yourself a favor and turn the volume ALL THE WAY OFF. It is awful. It is distracting from the gameplay which is otherwise pretty fun. It's interesting trying to learn the physics of this particular game to best control your ninja. And, when you lose control and hit a bomb and explode, it is hilarious. Plus lives seem to be limitless so that is awesome.
In other words: really fun, really great way to pass a short amount of time, really annoying to listen to.
Buy at your own risk. (I'd try the XBLA version to just see if you like it enough to carry it around with you. That's what I had played before I purchased the DS version and they are basically exactly the same.)