Nicktoons unite has some serious flaws which make it a huge disappointment. Fans deserve much better than this.
The four player mechanism is fine, but there are some serious flaws:
1) The controls feel "floaty" and rough - almost like something you would expect in a first build. Smashing stuff and shooting monsters is far from satisfying in this game. 2) There are basically no checkpoints... there is a "Goddard" point near the middle of each level, but for no good reason if you die you have to start the level, cut scene and all.
3) ...which brings us to the cut scenes. Yes, the original voice artists are used, but it sounds so "mailed" in it is almost painful. Basically a drawing of the characters face is shown with a dialog box while they speak...most of the time. The delay between lines ruins any chance of comic timing to work, and the lines are extremly unfunny. To make all this truly unbearable, you must watch the scene over and over again - there is no way to skip it. How Nick can allow this is unfortunate - especially when the quality of the scenes in Tak are so well done.
4) The special abilities also have that pre-release feel to them. You might need to open a door with a Spongebob squirt or a boy chin wonder chin-hand or a Danny Phantom walk through walls ability, but other than that the abilities are limited. Maybe the shrink ray that Jimmy Neutron gets is decent when you use it in battle sometimes.
5) Finally, one wonders why they made jump O and attack X when almost every platformer in the universe has X jump and  attack... everyone in my family was frustrated by this. So thats it - basically a typical game of its type, so it would depend on tight controls and funny cut scenes to make it worth while. Instead you get the frustration of having to start a level from the very beginning... and NOBODY in my family wanted to watch the cut scenes or have to replay the level. It's a real shame - if they had just put a few more months of development into this it might have been a decent game. My whole family loves these cartoons... but we never would have bothered playing past level 2 of this game except I went and bought the game and we felt an obligation to at least play it through one time.
If you have a kid who is just begging you to get this, rent it. They will definitely lose interest in this game long before its time to return it...