Lot's of fun, a great addition to an already outstanding game.
In general being a Vampire Lord is fun but I found I was seriously over powered, I think it would take a very silly combination of mistakes to ever die as a VL. In VL mode I was able to massacre whole dungeons without stopping and I don't think I had ever lost more than 50% of my health. But, did I mention how much fun it is to be a VL?
As a member of the Dawnguard you get a new weapon and cool armour. And you pretty much just hunt blood sucking Vampires. Which in itself is still pretty cool and loads of fun. But needless to say, I did enjoy being the all powerful VL more. I would love it if the VL could have a evil laugh emote so when I obliterate a whole room I could throw my head back a cackle an evil laugh.
The new content should give you 15 - 20 hours of great fun, possibly 30+ hours if you play through both the VL and Dawnguard quest-lines in two separate saves. Also, the new content comes with new areas to explore and a new mount, both are very well designed. Some new shouts and spells are thrown in to keep things fresh. It is not amazing or stand out by any means. But a solid, fun and enjoyable gaming experience.
I was quite happy to pay the money for the addition and would do so again without hesitation. The team at Bethesda deserve high praise for their continuing work on what I consider a RPG masterpiece. I await future additions with great anticipation.