Having just finished the expansion (PC version) on the Dawnguard side, my thoughts:

Good Stuff:
After a slow start, the new areas were all well-done and add a lot of lovely content to the game. Looks like the devs had a private biggest-cave-ever competition. The Snow Elf subplot was an intriguing piece of lore and fun to follow along. The new animal followers were fun to play with. The more advanced companion AI was nice (even if it was pretty silly for Serana to suddenly pull out a basket and start feeding chickens). The Soul Cairn, while repetitive, had a unique feel, just the sort of otherworldly place I was hoping to visit in expansion. The two-dragon combat on the frozen lake felt epic. The dwarven forge sidequest was ok and would be fun for a first-time player (I had already visited most of the places); also an interesting, if minor, lore addition. As a very small lore thing, I enjoyed that there were multiple, and conflicting, stories about who the original Dawnguard were.

Bad Stuff:
Good lord, was it glitchy. I had fine performance with the main game. With this running, I had frequent fps plunges and a couple of crashes to desktop when playing in the normal Skyrim areas. Perf was fine in the new areas. NPCs got stuck casting a spell forever. A dragon tried to nosedive into the ground (apparent bleedover AI from the Forgotten Vale lake dragons). A dragon skeleton suddenly appeared and I absorbed a dragon soul while fighting one of those random vampire attacks. And yes, I had the infamous Serana Wait bug, which is inexcusable to have after all the Xbox probs with it.

In terms of content, the plotline made no sense. Why do the vampires want to make the world hostile to humans, which they feed on? Why won't jarls believe the Dawnguard about vampires and raise armies when vampires are openly attacking cities, with their corpses laying there for easy examination? And why are the vampires openly attacking cities in the first place? Why didn't Serana and her mother just knock off Harkon in any of a million possible ways? Why do the vampires, who have an awesome castle, open its front door and run outside to fight when the Dawnguard attack? Why hasn't anyone ever noticed the gigantic castle full of vampires in the first place?

Also, it was utterly absurd that I would meet someone carrying an Elder Scroll on their back, have them say "I don't want to talk about it right now," and have no option to push harder or even physically take the scroll away.

I get that the whole point of this thing is to put more vampire stuff in the game, but I was hoping for more coherent story and less thinly veiled excuse.

Serana:
A powerful sidekick in combat. As a character, I found her endlessly irritating and not at all believable. She speaks like an immature, sarcastic teenager of modern America, not a centuries-old vampire of Tamriel. Even accepting that she could be emotionally frozen at her age as a vampire, it hardly made for compelling drama. Her preoccupation with her daddy issues rather than the pending apocalypse felt more like narcissism than a true internal struggle. Presumably she svcks blood from innocent people for food, but her dining habits conveniently are never discussed or displayed (she is not hungry even after being trapped in a hole for centuries) and the player can't ask about them. She's attractive, which doesn't overcome being bratty and whiny for me.

Middle-of-the-road Stuff:
Crossbows are fun but seem overpowered. Lots of new gear available--vampire stuff looks reasonably cool, while Dawnguard stuff is Skryim Brown and ugly. Fort Dawnguard has some neat features, including a trapped-filled cave/training area and armored war dogs, but no NPC ever noted them or explained them and I couldn't ask about them.

Overall, I'd rate it a solid C that needs patching. As always (at least for us PC players), the best part will come when modders use the new content.

I like Serana a lot, but it kinda svcks that my drain life in vampire lord form does diddly squat damage when shes my companion... I wish I knew how to fix that.

Yeah. You know I was wondering about these issues too. I mean, com'on! There's a HUGE castle filled with vamps right across the channel from a major Thalmor base? And no one notices the castle?

The Elder Scroll. They're treated like notebooks. While in the MQ (of the orignal game) the Elder Scroll found is a major issue. Has the power to send you back into time. I got the impression that Elder Scrolls are the foundation of the world. Of course, now I know they're just pieces of paper that any Moth Priest can read. Course, would've been nice to know this months ago.. could've save my char a lot of time.

The guy in Solitude who can rent you his boat to go the vampire castle says the castle is a ruin and is rumored to be cursed, that's why everyone avoids it. Still a bit of a weak excuse but maybe the vampires have other defences to hide them which are not explained further.

I agree made them seem alot cheaper, I could just see a used car salesman selling the DLC - "But wait you get not one but two extra Elder Scrolls if you buy it today"

The writing in Dawnguard was pretty terrible.

Yeah I can see people modding in all the other types of crossbow now they have the crossbow meshes.