Well I enjoy both the old and new cartoon, they both have their place, the original is supposed to be campy and a parody. The new one is grittier, relying more on stories, character development amongst other things to push it along, it is much darker than the original (though Fast Forward is less serious). The new TMNT has had some amazing stories and tight writing, especially in the later seasons (the animation is quite good as well).
Anyway I seen the movie last night and loved it, I still wish they would bring back Krang/Bebop/Rocksteady even if they made their characters more serious, but oh well at least it was a quality film reflection the new TMNT and the original comic in tone. My only big gripes about the film is that I wished they would have used the voice actors from the new show, Mikey was ok, and thought Raph was as well but Leo and Don seemed really off. Don't think I need to talk about splinter, I never could get used to his voice! The rest of the voice work was quite good though, Patrick Stewart was amazing as usual, Gellar was good as April, the guy who voiced Kasey was solid, and the one who voice Karai was quite good as well (similar to the new show).
The animation was wonderful in my opinion besides the humans who looked like they were borrowed from a Pixar movie. Most of the action scenes were well choreographed, and beautiful to look at (Leo vs Raph was amazing to look at with the rain), and the turtles themselves looked amazing.
Story wise, it had a good story but personally it had alot of holes in it that could have been filled, such as where the monsters have been all this time and why are they all of a sudden hanging out in the city? Besides that though it was a well written story with some wonderful dialogue here and there (Definately could see Laird's influence, like you can see in the new show).