The game may be the same, but everyone's different. So here's the question. How did you play/are playing through Dawnguard? Who did you side with, how'd you decide it, what was the impact on your gameplay as it was until now? What do you think you'll do in the next couple of times that you'll play? Was your RP affected, and how?
For the posts to be comprehensive, try and follow a layout like this:

1. Until I installed Dawnguard, I was (murdering for the Night Mother/done with everything/hunting for books and lore/other).
2. When the new questline kicked in I (sided with the humans and vampires/prayed on both until I decided/other).
3. While I was playing, I thought about (the new gameplay mechanics/changing sides/reloading a save prior to the DLC/other).
4. After the questline was over, I (assumed the new role I was given/kept doing what I was doing before that/other).
5. I wonder if (I will keep playing the same way/I'm ethically sane/Serana will get cured and die of Ataxia/other).

As for me, I was just done with the civil war, having sided with the Empire like the Dark Elf I am, and was about to join the Companions, while organizing the houses I'd purchased.

When the first vamp attacks occured, I went on to see Isran and get started (I can't remember ever siding with the "bad guys" in any game, but I thought a change was in order, so I let Harkon get all intimate with my neck. While I'm not really RP'ing, I generally like knowing where I stand from an in-game ethical viewpoint, which was devastated since I was both Tamriel's savior and at the same time an assassin, thief, extortionist, and who knows what else. Dwelling in an isolated castle, surrounded by vampires that are feasting on human cattle that's lying on blood-stained tables amidst gory gothic halls, certainly made my character look much less heroic than some of his prior choices indicated him to be.

Being a sneaky Destruction-Conjuration mage, with some knowledge of the other schools, I had no trouble in the new areas, and I've got to confess I didn't use the Vampire Lord transformation as much as I expected to (or thought I ought to). On the other hand, calling upon a couple of undead guys while throwing elemental projectiles at my enemies was much more interesting than floating around with only a handful of ways to kill stuff. So yeah, I can't say that being a dignified escort for Coldharbour's ladies thrilled me. I could as well be a regular elf (but an escort, nonetheless). Got to note that both the Soul Cairn and the Forgotten Vale brought me some good ol'Morrowind memories back, with the lack of directions and sense of artistic freedom that was present. Not to mention the breathtaking uhmm...let's call them cut-scenes, although that's quite different, but it's the closest I can think of.

In the end, after all was said and done and I was the new big bad wolf, I decided Vamp style doesn't really suit me, so I'll be trying to cure my self and see what happens. The reactions of the former undead comrades should be amusing, or at least weird. I don't know if they'd still accept me as the boss after being cured. But I'd rather abandon unlife for something much more earthly and cozy, like a mead hall brawl funded by a bunch of shape-shifters, whose quests I really want to enjoy.

I've only taken one character through Dawnguard. Prior to that she was a busy young woman. She turned her back on the dark brotherhood after the last main quest. Told them they were better of forgetting she existed or she would kill them all. She is the master of the thieves guild and checks on them from time to time. She still enjoys stealing but mainly fences off everything. She is a high ranking Legate for the empire having single handedly defended Whiterun and invaded and killed Ulfric. Even Alduin fell to her blade. But her greatest accomplishment and joy is the companions. The blood of the beast runs through her veins. Maybe because of that or just her nature but she hates vampires. Morthal became a better place the moment she stopped by. A few houses and nearby caves were emptied. Every night you can find her headed to Jorvaskr for a few drinks and maybe a fist fight or two.

So when she was approached by Durak, the answer was simple. "Kill vampires you say? Hell yes sign me up!"

But that kind of changes as she got further into the Dawnguard story. Her opinion of Serana at first was not a good one. I RP'd that my nord only agreed to take Serana back to her father was so that she could kill them all. That didn't work as she realized she was up to more then she could handle at the moment. Plus Harkon kicked her out with that magic spell of his. She fully planned on raising an army and going back to sink her werewolf fangs into his neck. But then Serana came back, spoke of her father's plans. My nord began to trust her, seeing that not all vampires were bad. Well most were... but not Serana. She went on through every possible quest to favor the dawnguard and destroy any last grip the vampires had. We all know how it ends. Had some memerable trips. The forgotten vale being the most interesting and beautiful place to date that she has discovered.

I can't see doing it differently. While all my characters have different personalities, they all prefer Skyrim the way it is. No matter how evil some of them are, they will always side with the dawnguard. I can't see playing a vampire and like Skyrim the way it is. full of life and sunshine.

Prior to dawnguard Veyalia was already quite a busy little vampiric dunmer (she contracted lycanthropy from the companions but quickly rid herself of that disease before gladly contracting vampirism), she quickly rose through the ranks of The Dark Brotherhood before deciding it would be quite a boon for The Listener to be a high ranking officer in The Imperial Legion. It wan't long until the vampire attacks came, curious to size up this preposterous "Dawnguard" she ventured off to see Isran; it was difficult to contain her laughter at the thought of the head of a group of vampire hunters sending a vampire on a mission. Before departing she knew she would be seeing Isran again, and he would not like her return. After meeting Serana and taking her back to her father things were looking better than ever. The rest is history .

There was one particular adventure she is quite proud of, tired of the overzealous Dawnguard continuously showing up Veyalia decided it was time to visit Fort Dawnguard again. After killing the woman patroling outside she quietly infiltrated the fort and turned every member she found in the fort into a vampire, including Isran; satisfied that the head of the dawnguard was now firmly in her control she casually walked out of the fort casting an evil grin towards the oblivious family camped outside.

Somehow i always end up playing good or atleast neutral with good tendecies
my first DC playthru , Octavia my imperial crusader was happyly , hunting dow every undead , daedra and other scum she could find
So when DG came around she immediately jumped on the DG bandwagon , didn't took long before Harkon was destroyed , now if only i can kill all vampires including that snottu Sera......