In my mind, superior to the 1987 show in many ways, Much closer to it's comic book heritage with it's own unique style.
Plenty of new stories, much more action than the original but more than anything the show goes into more depth, longer and more entertaining stories, a slighty darker cartoon than what your first impression may be.
The First season was Great, a new (and better) look great plot and we end the season on a cliffhanger with the turtles being transported through space but before all that we see great battles with the Shredder, Casey jones gets the role that he should have got in the 1987 cartoon and then we have several new bad guys like "Garbage man" and "The Purple Dragons" "Hun" and several more I meen c'mon what else do you need.
Season two we go even further with the turtles landing on a ditant planet and end up in the middle of a Galactic war trying to save "Prof. Honeycutt", not to mention we learn that Shredder is actually an Utrom and then to end the season with The battle nexus was a stroke of genius.
Season three showed that even after two seasons a cartoon can keep getting better, we meet Agent Bishop who try's disecting the turtles but is stopped just in time and overall becomes one of the main bad guys in the show, we also see Shredder in what would be one of his last atempts at total control over the galaxy as he is stopped once again by the TMNT.
Whilst not my personal favorite season it still manages to keep the good times coming.
I can't really comment on season 4 and Season 5 (Ninja Tribunal) as I have not seen all that many episoeds of them.
Season 6 (Fast Forward) the TMNT are transported through time to the year 2105 and meet Cody Jones a decendent of Casey and April, the show took a definate turn here in my opinion whilst still enjoyable it had lost it's edge even with alot new enimies it still felt unneeded and I believe I am not the only one who will be sorry to see it go.
It is much closer to the 1987 cartoon than any other series whils it is more funny than the last five seasons it had lost it's gripping plots and darkness which is what made TMNT 2003 so much fun to watch.
All in all I just don't know what the producers are going to do, personally I think they should just retrace there steps and make the cartoon what it was from the first couple of seasons but even though with the recent mess up that is Fast Forward I am going to give TMNT 2003 9/10.