The Good
I thoroughly enjoyed playing on the vampire side of things as my first character, the vampire lord looks badass in a way that cannot properly be described.
The characters of the Volkihar vampires are interesting and I loved their castle, had a unique feel to it that I hadn't felt in Fort Dawnguard. The 2 new areas, The Soul Cairn and The Forgotten Vale are great additions, The Soul Cairn is probably the most unique looking place in skyrim and even though it may appear empty and samey it made me want to explore it more, which is certainly saying something as I hadn't ever felt that way about a location before. The Forgotten Vale is just... in a word... beautiful... I never thought I would see such a beautiful place in a game that is not only pretty to look at but is also expansive and interesting. Serena is also one of the best characters in skyrim. Funny, charming and suprisingly attractive she is great to have walking about with you and she is a welcome addition to the memorable followers list.
The new equipment are certainly interesting additions. The Crossbows are good fun and are quite powerful for even low level archers. The new armor such as the Falmer Heavy Armor and the vampire armor not only look great but also have decent stats attached to them.
Finishing off is the great attention to detail and, to put it quite simply, the best side quest in Skryim, "Lost to the ages" if you haven't tried this wonderful side quest just do it, challenging and rewarding, just how I like it.
The Bad
During the course of the DLC i never really felt as though there was a proper war going on between the Vamprires and the Dawnguard, sure the Dawnguard pop up from time to time but even during the main quest of the DLC there was no epic battle between the factions, which was a big dissapointment for me.
One thing that really bugged me is that even while playing as the Vampires, you killed loads of vampires... and gargoyles... and undead... it really didnt make sense to me how many of my own badass kind I was killing, it would have made sense for the Dawnguard to be killing this many creatures but even as a vampire it confused me how much of my own kind I could happily kill. This also extends into how much you can probably tell I love the vampires side, the Dawnguard however... not so much. The characters are relatively generic, their fort doesnt feel like a fort due to cobwebs, clutter, emptyness ect, there doesn't actually seem to be that much of an 'army' so as to speak as instead there being about 5 people, who aren't very interesting. Finally the fact that the 2 faction's main quests are basically the same. If I got my own way of how the Dawnguard Main Quest went I would have made an assault on Castle Volkihar happen at some point and maybe to do so (and I know this is generic and sad so don't judge me) the Dawnguard recruit werewolves to act as cattle for the Volkihar (seeing as one of the Thrallmaster Vampire mentions that some people come willingly) and then when the Dawnguard attacked all hell would break loose and the werewolves would help tear the vampires apart, but hey thats just my 2 cents.
The vampire lord form is relatively clunky and slow and while it can be powerful werewolf seems to overpower it again on the end scale of their skill trees. Vampire attacks can sometimes become annoying, Auriel's bow i believe needs a bit of a damage boost considering it is a weapon owned by a god. I wish I could invest in the castle and the fort by paying to upgrade them and so on so forth with nitpicking.
And once again, bugs, I know this is bethesda but during my first playthrough the slowdown in castle volkihar was terrible, the game crashed on me 3 times, Serena suddenly went beserk and attacked everyone in castle volkihar for no reason and at times various quests wouldn't continue. Along with that the Serena Drain Health Ball glitch didn't get fixed and as a result many people don't realise this is a glitch and just consider the vampire lord to be terrible in terms of damage. The beta test I believe should be sent out slightly earlier so that Bethesda could have a bit more time to iron out these bugs but of course I don't work at a games company so I don't know how these things work exactly.
The Verdict
Dawnguard is a great addition to Skyrim and a wonderful first DLC. With memorable characters and locations, excellent main and side quests and a slew of new equipment it definitely holds itself as a proper expansion. While various features don't feel completely as polished as they should and bugs continue to creep in on a large scale if you can overlook these flaws you can see Dawnguard as a fantastic start to Skyrim's DLC roster.