It's not so much that they came in later, as much as they come in later, disappear and come back even later then... well... The Turtles should have been in more of their own movie. I'm actually wondering if giving it a shorter runtime than a Transformers movie was a good call or not. Cuz, at least in a Transformers movie, when the Transformers arrive, they stay in the picture. And the bloated runtime actually makes room for them, even though the humans still have too much darn focus.
But the second TMNT could be promising.
I don't think it's a bad movie, so much as a meh film. I did miss the beginning, so I must have lost something. But overall, it was okay, I guess. Maybe stronger than some of their other movies, but not by much. Robots was more deserving of a sequel.