Naruto was a manga that started with the potential to be one or even THE best shonen ever made. It had everything that it takes for a success: a vast quantity of well-made characters, an interesting setting (the shinobi world Kishimoto designed is awesome), interesting villains (for me Orochimaru is a copy of Voldemort, but it was still good) and a solid premise.
Even though the series is named after the main character, it is the supporting characters that really give life to "Naruto": Kakashi, Rock Lee, Gaara, Shikamaru, Hinata, Itachi, Minato, Tsunade, just to name a few. They are all interesting characters. Some of them even got their own "fan-clubs" during the serialization of "Naruto".
But at some point around volume 45~50, the story quality just started to decay greatly. I think Kishimoto didn't know exactly how to tie off the knots he himself created in the plot (similar to the way GRR Martin doesn't know how to wrap up A Song of Ice and Fire, but that's another story). The plot became too big, and apparently Kishimoto wasn't able to deal with it.
I still give it a 9, given the great quality of the first 2/3.
I started watching Naruto in 2006, when I was 11 years old, and I started reading the manga in 2009. "Naruto" was a part of my teenage years. Maybe that's why I still give it a good grade despite its unsatisfying ending.