This review will contain spoilers!
In many sentences:
I must mention that I saw the animated film version of this prior to reading the book, though it had been at least a few years. Still, I knew, generally, what was going to happen so there weren't a lot of spoilers for me in that regard. I plan on watching the movie again, now that I've read that book, but overall I think the movie did an excellent job of capturing the sense of wonder and magic that permeates this book.
I really enjoyed Howl's. It was funny and charming. The characters all felt alive, and everything worked together perfectly. From the tongue in cheek chapter titles (i.e. In Which There is Far Too Much Washing) to the way it ended with everyone clamoring for Howl and Sophie's attention, while they remained oblivious, and continued to argue with each other.
I also enjoyed how subtle and assuming the book is about pretty major facts. For example, at the end, Sophie turns back into her young self, but the book doesn't really tell you. It makes a few mentions of how her hair was red again, and that's pretty much it. It never goes into great detail about why she was cursed (she did it to herself, most of the time), but lets the briefest of comments suffice. This was refreshing, especially after reading so many young adult books that have to explain and justify every single character's actions.
Another example of this was the fact that Sophie and Howl were in love. Honestly, it all happens so quickly, and with so little exposition, that I wonder if I would have realized it if not for my experience with the movie. I think I would, because Sophie's jealousy does grow slowly over time, but I admit it's possible I may have missed it entirely, until the very end.
Ultimately, Howl's is a magical little book that I recommend to anyone who likes fantasy. It is heart-warming and high-spirited. Too often the best books are depressing, so I love when I come across a book that is both excellent, and uplifting, without feeling manipulative or cheesy.