If you have a bounty in Riften, the Dawnguard will simply attack you with no explanation as to why. If you go pay off your bounty, then they won't attack you on sight.
Completed the main content in Dawnguard, took me about seven hours. There's still a bunch of miscellaneous activities to complete but I don't feel much drive to do them. There's one questline that I haven't stumbled across, I can chase down the two other words to a shout, use a weapon, fight a specific dragon, change my appearance and grind through the Werewolf and Vampire skill trees. At least becoming a vampire lord looks like it's still available though through a follower I now have.
All in all I would have to say Dawnguard was pretty disappointing. I waited nearly six months for that, I had completed Skyrim about as thoroughly as one could and Dawnguard really didn't add enough, not to justify its price. If you're intent on buying it, I'd say 5-10$. I'll try to hammer through the rest of what Dawnguard added but I don't expect it will drastically change my opinion.