I have not seen the play so I cannot comment on differences between it and the film, for better or worse.
I saw War Horse this afternoon and it was very crowded. I would guess it was just about sold out, not more than a dozen or so unsold seats.
The story felt sort of contrived but it is a British children's story, correct? If so, some of the glaring, annoying coincidences make perfect sense if they're intended to be read/heard by a child. That said, I was able to put most of my quibbles with the plot itself aside and enjoy the film. The cinematography was beautiful and I thought the soundtrack was incredible. I didn't think any of the acting was particularly noteworthy, but the plot and sequence of the film doesn't really allow for any actor to shine.
When I left the theater, I certainly felt a bit of "magic." It was an enjoyable movie. Perhaps not brilliant nor deserving of The Best Picture title (though a nom is probably imminent and well-deserved), the movie really draws you in to the excitement, evoking a sense of wonder and amazement many of us haven't felt since we were children, if ever. I'd highly recommend a visit.
Scratch and claw for every day you're worth!
Make them drag you screaming from life, keep dreaming
You'll live forever here on earth.