I guess this should actually be called Book vs. TV Show, but since it's the one and only of its kind (so far) I'll just include it with the movies.
So like any good reader, I read both A Game of Thrones and A Clash of Kings before watching seasons 1 and 2 of the HBO fantasy drama Game of Thrones. I actually started watching the first season while I was still reading the first book, so it was cool to see the characters come to life almost immediately after reading it.
I think that Game of Thrones is the best book to visual medium I've ever encountered. HBO spares no expense when it comes to costumes, sets, and locations. They did a magnificent job casting the actors, so everyone looks and sounds pretty much as I imagined them to. The script is wonderful and nothing is left out. I know the real reason is because they dedicated an entire season to one book. It's like making a ten hour movie. Literally, everything that happens is included and there are even scenes that have been added to enhance the viewing experience and to help non-readers understand what's going on.
In the first season nothing was changed, but in the second season there were some changes. Dany's, Jon's and Robb's storylines are altered, but I didn't mind too much because it gets all the characters to the same place in the end.
Overall, I think HBO did an excellent job with the adaptation (it helps that the author, George R.R. Martin, is a producer) and I'm looking forward to the third season! So if you read A Song of Ice and Fire or are just looking for a good show to watch, I highly recommend Game of Thrones.