As the Dawnguard DLC opens up, you're invited to join the Dawnguard, a group of vampire hunters. After completing a couple of introductory quests for them, you learn that an ancient family of vampires is trying to fulfill a prophecy that would give them power over the sun -- something that would spell doom for all of the non-vampires out there. At that point you're given the choice of joining the vampires or sticking with the Dawnguard, but surprisingly this decision doesn't make a lot of difference, and the campaign plays out about the same (if you join the vampires then you decide that the head vampire is too obsessed with the prophecy, and you turn against him).
The DLC has some nice touches. There are coffins for you to sleep in. There are raging rivers that send you rushing into new zones. There are frost giants that seem to have a fetish for important gems. There are large, intricately designed outdoor areas, including the Soul Cairn, the place where souls captured by soul stones go. There are armored troll pets. There are crossbows. There are new crafting recipes, including a few for dragonbone weapons. There's a face sculptor who can change your appearance.
These additions, along with a 12-quest campaign, might sound like a lot of nice stuff for a DLC, and perhaps it is, but I felt underwhelmed. The first problem is that almost all of the additions are cosmetic. The Soul Cairn is an interesting place to visit, and Bethesda did a nice job in making it feel bleak and forlorn, but exploring it is no different than exploring anywhere else. Meanwhile, frost giants are the same as regular giants, coffins are the same as beds, crossbows are the same as bows, and so forth.
The other problem with the DLC is that its new big attraction, the ability to transform into a vampire lord, is worthless. Just like the werewolf form before it, unless you're level 20 or something, you're going to do way more damage and have way more options with your regular form than with either of the specialty forms, and so there isn't any reason to use them, except perhaps for role-playing purposes. Bethesda tried to make the forms more powerful by adding perk constellations to them (you gain perks by killing enemies while using the form), but even with the perks the forms are underpowered.
The campaign also has some problems. I already mentioned that the big decision you have to make doesn't change anything, but there are other issues as well. For example, I'm pretty sure the enemy you fight the most in the campaign is the falmer, which seems like an odd choice. The campaign also introduces an evil vampire named Harkon, but he spends 95% of the time off screen, and the only reason you decide he's evil is because of stuff you hear secondhand. Harkon doesn't actually do anything evil. He just hangs out in a castle. And finally, while the campaign is active, there are random vampire attacks in Skyrim's major cities. This is great for making it feel like there's really a vampire conflict going on, but the vampires far too easily kill named NPCs, including shopkeepers and quest givers, and so their attacks are more aggravating than anything else.
I played through the campaign twice, once with a level 30 character, and once with a level 53 character who had finished the game. The level 30 character had some difficulties towards the end, but the level 53 character had no trouble whatsoever. I'm guessing everyone who buys Dawnguard has a level 50+ character sitting around with nothing to do, but the DLC isnâ€™t the answer. If the content scales at all, I didn't notice it, and it seems most appropriate for middle-level characters.
The Dawnguard DLC is priced at $20, and it might take you somewhere around 10-15 hours to complete it (depending on how much you like to go on radiant busywork quests). I didn't find much in the DLC that I really liked, but it's not terrible, either. The production values are good, and there's lots of new content, but nothing about the DLC really feels different than what the game offered before. And so Dawnguard is a DLC that you could buy or not, as you see fit. I'd only recommend it to those of you who have explored every dwemer ruin in Skyrim and who desperately want more.