I really like FF8, but I don't think its underrated - not like poor FF9 or FF5 or anything like that. Most people who dislike FF8 still admit that the graphics were amazing compared to 7, and that the soundtrack is one of the best FF soundtracks. However, most detractors had a problem with the plotline, which I agree is realtively weak compared to some other games:
1. Ulty came in so late in the game (begining of disc 3 was the first time we'd even really heard of her), that she was confusing as a villian. Sure, Edea being possessed was somewhat of a switch, but they dragged that on too long. To really hate an enemy you have to know who they are.
2. FF8 gave us some awesome characters, like Quistis, Irvine, Seifer, etc, but focused so much on Squall and Rinoa that we never really got to explore these other characters as much as we should have.
3. Even I thought the sudden "oh my God, we all grew up together as kids and forgot!" thing was kind of lame. They should have played up Squall and Ellone's heritage more instead - that was much more interesting and surprising, and fit with the "Fate" theme.

Long story short - its a great game, and doesn't deserve a lot of the crap it gets, but I don't think its underrated. It's still one of the most popular FFs.