Of course it's not the book, but what screen adaptation ever is? There's no way a work of over 800 pages and full of digressions can be adequately condensed into a two-hour film. As some people have said in the Message Boards section, we should be grateful that the film will make this neglected masterpiece known to a wider audience.
So Aranda does what so many other screenwriters have done in the circumstances: chooses one incident or aspect out of a multitude, elaborates on that and tries to remain faithful to the spirit of the book. But does he remain faithful? That's the trouble. Certainly there's a love story with sensuality in the Tirant and not the platonic stuff of other mediaeval romances, but Aranda piles on the sex until it becomes a bawdy romp. It's a pity that people will get the impression the whole book is like that.
That said, the acting is good, the hero is handsome, the heroine is delicious dressed and undressed, Yvonne Blake's costume designs are truly gorgeous, and the film has humour - in fact it could be classed as a romantic comedy. I especially liked the ending, but no spoilers! You can also learn a bit of romanticised history about the Turks and the end of the Byzantine Empire. (Tirant goes to Constantinople to defend it, but truth is that Constantinople had already fallen before the book was written.)
A special tribute to the dubbers is in order. I saw the version in Valencian, and the dubbing was so perfect that it could well have been the original.