Twilight Princess is a great video game, but a below average Zelda game. I do not believe that it is overrated, because this is how most Zelda fans seem to see it.
On the whole, the game was quite easy. This was good for me on my first play through as I had hardly played a Zelda game before (I had started Ocarina of Time but had only gotten to Dodongo's Cavern), however, it made it less interesting to replay after having experience with it.
On the whole, the enemies were not all too difficult defeat, excepting to some extent the Darknuts, but once you understood the pattern, the were not terribly difficult either. Even Stalfos, an enemy that in past games such as Ocarina of Time had been somewhat of a challenge, especially on 3-Heart Runs, are really no trouble to defeat.
Also, the bosses were predictable and rather simple to defeat, especially for one experienced in Zelda. All of them required use of the dungeon item and usually that item alone and had common weak points, such as the eye. There were a couple of creative bosses, namely Argorok and Stallord, but they were still easily defeated, just not by traditional Zelda means.
The dungeons are also not all that complicated, especially since many were based on concepts from Ocarina of Time, or were just simple dungeons. The Forest Temple and Goron Mines were both simple, straightforward, start-of-the-game dungeons. The Lakebed Temple that followed was little more than a watered-down (no pun intended) version of the Water Temple. After that was Arbiter's Grounds, which involved tracking down Poes, essentially like the Forest Temple from [Ocarina of Time[/I], except senses made it easier. Snowpeak Ruins and City in the Sky had some new, creative aspects, but those were offset by Temple of Time and Palace of Twilight being basic and simple.
On the whole, Twilight Princess was not a difficult game to play, and there were several boring aspects that made things uninteresting for playing the game a second time (such as the 45 minutes+ time before the 1st dungeon). However, all things considered, it was a very good game. The Wolf transformation aspect was not revolutionary, but something that was interesting in some places. The graphics looked very nice in my opinion, and some of the Hyrule Field sequences, especially those involving King Bulbin, which were over world boss fights not seen before in the Zelda series, at least not to that scale. On the whole, Twilight Princess is an amazing game, it just cannot live up to other games in the series.
So, Twilight Princess is not an underrated game, I find that most Zelda fans who have been fans of the series for a long time or who have played most of the games rate it almost exactly correctly.
I think every Zelda game is overrated if you think about it.
