My dd took AP World hist as a sophmore and AP Us Hist and Art History as a Junior (she took 4 others these past 2 weeks - but I can't tell you about those till her scores come out).
She scored 5s on them all - and all were just done at home. We didn't even have a curriculum for Art History - she read Annotated Mona Lisa, she worked through a book Walsh puts out on 35 artists, she watched Sister Wendy videos, we wtahced Teaching Company videos on teh Louvre and then visited the Louvre (that was a nice field trip!!) - and then we foudn a AP test prep book on Amazon that had just come out and it had a CD with it with lots of pictures of art works.
She loved studying for it, but felt there were big holes in her knowledge too. And she came back from the exam saying it was a nightmare - she'd hardly recognized any art works they'd given. So she guessed wildly and made things up. She is a good test taker and would have made educated guesses, but I would still say this can't be an extremely hard one to score well at as she got a 5. That said though, one of her friends who is at school did it as a school course and managed a 1!
So, I guess my advice is - if she likes art, I would do it. Any AP looks good when applying for scholarships!
Meryl