Its really good. Its a refreshing new place to visit with interesting groups to meet and things to do. From the other Dawnguard people I've heard this one was much better.
It brings a new view to Skyrim. Instead of everything being snowy or the world is clear to view now the ashen landscape brings something different to the table. Not so much a forboding feeling but a slightly alien feeling to the landscape. The creatures are pretty interesting as well as the people and crafts there. The enemies are actually decent and there are quite a few interesting things to discover.
I don't really know much about Dawnguard as I've never played it but this game was pretty good.