I really ended up enjoying this series quite a bit. Although the beginning was a little slow going for me, the series picks up around the sixth volume, and continues to improve as the story unfolds. I would rate the first third three stars, the second third four stars, and the last third five stars. Over time I became fully invested in the characters and their story lines, even shedding a few tears at the end. I thought the romantic relationships and the love triangle in particular were handled well. Fruits Basket is able to avoid the very common problem of predictability. The resolution of the love triangle is not obvious before the actual resolution. I also enjoyed the friendship relationships between the characters, and the character development, which happens slowly and organically over the course of the series. Self-sacrifice is a very strong theme in this series, as is acceptance of others, and grief and loss.
Note: The anime faithfully follows the first third of the manga, but has a completely different ending. Because two thirds of the series is not covered by the anime, I'd recommend the manga over the anime, particularly because I thought the latter two thirds was better than the first third. For those who have already watched the anime, the manga is still worth the read.