actually in a in a really horrible way i though umbridge was one of the best characters in OotP. annoying, evil and cruel yes. but so well written i loved all the little touches like her "hem hem" and her constant description of being toadlike. obviously you don't like her but she was a really good villain and i thought most of her bits, paticularly with mcgonagall were hilarious. i detested her obviously but you enjoyed hating her, just to see her get her comuppence.
(that's evil laughter by the way)
i actually found grawp to be a bit dry and boring, i like hagrid but i often find the hagrid storylines like norbert in P/S and you know his scrapes and stuff,, if they go on for too long, i switch off so i didn't really like the grawp storyline, although it will obviously have some significance to the plot in further books and will affect the outcome of the war in some way, i predict anyway.