Its really good. As ebert said, its the best possible movie they could have made with the source material. Its genuinely funny and touching at times. Shredder is a legitimate badass, and I'll say the crazy, I would argue they somehow made a believable plot/world with the scenarios.
Addictively quotable too.
"Cricket? No one understands Cricket! You gotta understand what a Crumpet is to understand Cricket!"
"Wise man say 'forgiveness is divine' but never pay full price for late pizza"
"Radical Radical Radical"
"I made a funny! Hahahaha"
...
"All fathers... love their sons"

Only thing that hasn't aged well are some of the fight scenes.