The interesting concept will amuse you, but the lack of goals is likely to keep that amusement short.
You play as Boy. Boy is a... creature that begins as 2 spheres joined together, a couple of legs on each, and a face plastered on one of those spheres. Each of your analog sticks controls one end of Boy. Thus pulling in separate directions stretches Boy. After a brief moment of elasticity, Boy starts to grow new segments in the middle, and gets longer. It's possible to stretch yourself across an entire stage and drape over the edge of them, and you can also shrink yourself at any time. There are no enemies in Noby Noby Boy, but each world is populated by various objects, and it is initially amusing to stretch your body in different directions and wrap yourself around some of the larger objects. You can also interact with objects by eating them if your head is large enough. This is where the game is a bit confusing, as I'm not sure what actually makes your head bigger. I assume this is from eating, but when I shrunk my body down to the starting size, sometimes my head was bigger than my tail, other times it was normal size. Thus some items can not be eaten until your head is big enough. If you eat something when you haven't stretched yet, you will poop it straight out. You will also poop out an object if you have stretched, but can't hold any more objects in your body.
And really, that is Noby Noby Boy in a nutshell. There are no obstacles and no goals, just a virtual playground and some physics to mess around with. Apparently eating some objects in succession means you will poop out a combined object, but I never noticed it, and objects can fly away so fast you don't really get the opportunity to see them anyway. While you do have control over it, the camera can be a bit troublesome at times. You might zoom it out to a desirable level, but then it may autozoom again based on your movements, or not be highlighting the part of your body that you want. The worlds are all randomly generated (and you can load a new map at any time), and some of the charm is seeing just what will turn up next. There are people roaming around (and will sometimes jump on your back for a ride if you are stretched), sometimes there will be giant mushrooms, sometimes people will drive around in cars, jungle gyms will appear to wrap yourself around, or windmill type constructs to get tangled in.
The graphics and sounds are all on a very basic level, but work as part of the games charm instead of against it. It is obvious that this came from the designer of Katamari, and a similar style is displayed here. Each part of your body can also jump, and by having both jump you can effectively fly up to the clouds and drape yourself over them if you desire. The jump button is also the same as the eat button, so it can be annoying to try and eat something and jump instead because you weren't placed exactly where you thought you were. The physics are interesting to play around with, and it can be fun to see your back half of your body slingshotting around while you only move the front around, trying to tie yourself into a knot, or watching your body fly about from explosive pooping after you shrink yourself while you have a full belly.
There is one overarching goal in Noby Noby Boy, and that is to grow Girl. This represents the online component of the game. The game records how much you have stretched, and this is added to a tally comprised of every online player in the world. At the outset of the game, Girl grows towards the moon, and has since also reached Mars. When these are unlocked, you can load maps on these worlds, with new objects that are unlocked. This is an interesting concept to make players feel like they are contributing to a larger goal, but it seems like the going is slow and only dedicated players who have worked out strategies to stretch while they aren't at their system are the main contributors. You can also play two players on the same system, which is fun for a short while as you can eat each other and poop each other out, but the camera focuses on player 1 and it's issues are exacerbated here.
Noby Noby Boy is an interesting concept, but is more like a virtual toy than an actual game. While you may only get an hour or two's enjoyment out of it, it's cheap price means it won't feel wasted. If you don't want to get stressed while gaming and just want to relax, then you may get more longevity out of Noby Noby Boy.