Legend of Zelda is my favorite video game series, and Wind Waker is definitely a solid entry relative to the series. I actually played it before playing Ocarina of Time (The first Zelda game I played was A Link To The Past, then the Oracle games on Gameboy, then Wind Waker). I was 12 or so when I first played it. I never had a problem with the art style myself, possibly partially because I didn't have the expectation from playing OoT or getting hyped over the Zelda tech demo at Spaceworld. I was somewhat aware that it was a point of contention with some fans online, and I had at least one friend who was turned off from the game because of the art style, but I always liked it.
I don't think that the game is underrated, because I get the sense that it's rated pretty highly. There are plenty of reviews that are effusive in their praise of the game, and I don't think I've seen a review that really harshly criticized it. It seems to me that a lot of the fans who disliked the art style grew to like it once the game actually game out and they saw how beautiful it actually made the characters and the world. There are some holdouts who still don't like the style, but they feel like a minority to me. Some people dislike how long sailing takes or how easy the game is, but thankfully Wind Waker HD took steps to remedy those criticisms. You can get the Quick Sail to speed up sailing, turn on Hero Mode to double your damage taken and eliminate recovery hearts thus making the game much harder. It also streamlined tracking down Triforce pieces towards the end of the game, which was a drag for many people (including myself). I will say that if you're looking for the best dungeon crawling experience in Legend of Zelda, Wind Waker isn't quite it, because it only really has 5 full dungeons and 2 pseudo-dungeons. So I can get people being underwhelmed about the game on that aspect.
Relative to the other Zelda games I've played, it's definitely not my favorite, because that spot is reserved for A Link To The Past until the end of time. It's hard to compare it to Ocarina of Time and Majora's Mask, because I think they're all great games. I will say I think it's a more memorable and distinct game than Twilight Princess, both for the art style and the narrative. Wind Waker does some neat things with playing with your expectations as a Zelda game. After a couple games with the "find three artifacts in three dungeons, get the Master Sword" pattern, it looks like you're doing the same thing in Wind Waker -- until you find that Ganon attacked the holder of the third artifact, and you get it without actually going through a dungeon. When you get the Master Sword, you figure hey, why not go attack Ganon now -- turns out, the Master Sword has been depowered and is actually worthless against Ganon. It just felt a bit more creative and unpredictable. Whereas Twilight Princess follows the formula of A Link To The Past and Twilight Princess basically to a tee.