Teenage Mutant Ninja Turtles has been around for over a century, and there were some people who got worried about 4Kids owning it's license, but now, even from people who generally hate 4Kids Entertainment, people believe it's the best work they've ever done.
Let's start with the writing. It's good. There are several seasons, story arcs, and even a few times they reference the 1980's series (Especially in that "Secret Origins arch. They referenced it 3 times!). This is actually darker than that old series, but still keep it to a TVY7 rating. My only real writing gripe is saying "shell" instead of hell. It's clever, but I just hear it too often for it to be that clever. Now it's just annoying. None of the turtles say anything out of character, no terrible puns, and nothing really cheesy. Plus almost every character has some kind of backstory that they reveal to you in time, which really brings out their character.
The animimation is what I will call "very good". It's no spectacular, but it's way above average. Nothing is ever choppy, they're smooth and accurate, and they have correct speed (by that I mean fast blows really have an impact and slow blows are weak). Even in some of the chaotic fighting scenes the animation doesn't slow down. Good work there.
The music is the kind of music you'd want to listen to while playing an extreme beat-em-up game, which is a good thing. 4Kids is really good with the music and theme songs, but the music IN the show could use more guitar, but that's just my tastes. The voice acting is as great as you'd expect. Though sometimes they go a little off character when they voice. Splinter's voice also sounds a little breathy, like he's exhaling air more than he's talking.
Let's wrap this up with my opinions on the series' characters. I just love them. All of them are very unique with their own set of abilities. You've got Casey with his blunt objects, Shredder with his claws and technology (plus he's a badass), and my favorite, Baxter Stockman. Super genius. And with other unique villains and heroes it makes it look like they'll never run out of ideas.