I thought it started out fine, but then became less and less interested only a few episodes into the first season because it is somewhat biased towards Leo and Donnie. The episodes often focused on them, and they were usually treated as the most important characters. If the 1990 movie is any indication, Raph used to be an important character as well.
Whenever they dodged attacks, I disliked the fact that Donnie would usually jump in the same direction as Leo while Raph would jump in the same direction as Mikey. Whenever they split up, Donnie would usually be with Leo and Raph would be with Mikey. There was at least one episode (season 1 I think?), probably more, where Leo saved the day and Donnie helped in some way while Raph and Mikey just stood around and watched, which doesn't make much sense since Raph is the one that likes fighting and you'd think he would be the one that would help Leo. There was that season 2 episode were Mikey is unconscious, and Leo tells Raph to look after Mikey and Donnie to go with him and fight (what a surprise). But what I disliked most about this series was that episode where Donnie becomes a monster. This happened to Raph in the comics, but I guess they felt the need to change it in order to (once again) keep the focus on Leo and Donnie and have Raph standing beside Mikey all the time and being treated like a less important character. Last but not least, there's that Fast Forward episode where Mikey becomes a monster then later Leo, Donnie, and Splinter turned into monsters as well. Near the end of the episode, there is a close-up shot that just has Leo, Donnie, and Splinter together as monsters but not Mikey. I know Mikey is the Comic Relief and that Raph was the one that had to save the day in that episode, but come on.
Overall I don't hate the series, and I still have a VHS tape that has a recording of the first episode from when it aired in February 2003. I just wish Raph was a more important character than he turned out to be.