Something to consider when measuring Naruto's power:

1) Kyuubi hates Naruto. He wants nothing more than Naruto to suffer like he is suffering, being imprisoned inside a mere human's body. The only reason he ever lends Naruto his power is when Naruto is in a life threatening situation, because as was stated in the series, if Naruto dies then Kyuubi dies. Kyuubi only cares about self preservation, and in the long run escaping imprisonment OR finding a way to take over Naruto's body and claim it as his own.

2) In most cases, Naruto's ability to mold chakra is pathetic. He is constantly wasting chakra attempting to force jutsus that his teamates, Sasuke and Sakura, could perform with a mere fraction of that chakra. While Kyuubi's huge chakra makes Naruto potentially possess Sannin lvl power, he can barely control his own chakra, let alone Kyuubi's. Because of this, he can only use it in the most primitive ways, forcing it to his limbs to boost his strength, speed, and resilence, while most of it bleeds out in a form of visible chakra.

3) In order to maximize his tap into Kyuubi's power, Naruto basically has to lose control of himself to Kyuubi's beastial urges. I wouldn't be surprised if after his latest transformation, a later story arc deals with a conflict between Naruto and Kyuubi over control of Naruto's body and soul. Kakashi and the others do not freak out and begin trying to subdue Naruto when Kyuubi's power begins to spill forth for nothing.

Naruto possesses potential to be the strongest character in the series, with his dedication to training and his attitude towards life. He also has a huge amount of power that he is barely beginning to learn how to control in the form of Kyuubi's chakra. Imagine if someone on the lskill evel of a Sannin possessed the sealed Kyuubi, that person would be able to single handedly destroy entire villages. Which is precisely why Akamutsi(sp?) wants Naruto and the sealed Kyuubi for themselves.

But as it stands now, there is a huge amount of characters in the series that could wipe the floor with Naruto, Kyuubi or not. So no, I don't believe Naruto is overpowered at all, in fact I believe his is one of the weaker characters in the series as he is now. But that is a huge part of the suspense in Naruto, there are many characters stronger than Naruto and his friends, so theres no feeling that Naruto is going to just whoop whoever shows up and use their bones for toothpicks. Every encounter is filled with a sense of danger, to Naruto and his companions.