I liked 54 a lot. I liked the ending, and pretty much everything, although I do understand people's frustration with the jumping around and out-of-the-blue villain. The rushing around I attribute to the limited page count, and I think KAA covers the essentials and I don't mind filling in the blanks myself. The One, now, I'm not so sure about. I likehow it ends with Jake's order to ram the ship, and how they went off on this new adventure, and how it's open to interpretation. But The One, itself? Ehh.
I disagree that Rachel should have died sooner. As the books get progressively darker throughout the series, it makes sense for one of the protagonists to die in the end. Masterz1337 mentions the team falling apart after her death and the end of the war, and think about it, if Rachel died sooner, Tobias would have called it quits, and I'm not sure how Jake would have coped. I don't mind that she died for apparently no reason, either. She still fulfills her role as being the ruthless warrior in killing Tom--Jake couldn't have asked anyone else to do this--and hey, even the grandest plans go awry. I mean, it wouldn't be an Animorphs mission if something didn't go horribly wrong. As the Ellimist tells her, she still mattered.
I wouldn't have minded if Tom lived. Or maybe I'd have just been fine if we saw something from his perspective just once. I really want to know what he thought when he found out about Jake because the idea just warms my heart.
About Tobias being unhappy--I'm with KAA on this. I don't think Tobias has much of a will to be happy after Rachel's death--contentment with a simple existence, maybe, but happiness is a human thing, isn't it? And he doesn't want anything to do with human society after the war. So yes, in a sense, he deserves to be unhappy because he's making himself out to be. Jake is pretty much the same until he starts to get his **** together, and even then he's still obviously damaged (how much do I love his conversation with Marco about redeeming himself and Marco calling him on his issues? So much.)