-Quick Review-
Dawnguard was the first official DLC that Bethesda released in 2012, you gain an additional quest line, cross bows, Serana as a follower, new magic spells, Werewolf and vampire perks, new locations, and more.
http://steamcommunity.com/sharedfiles/filedetails/?id=517147037


-Detailed breakdown review-
Story: Long story short; you join up with the Dawnguard to hunt vampires and protect the land, the story splits and you get to make a decision early in the story that changes how the story preceeds.
I actually didn't realize I was playing the DLC at first. (I purchased the Legendary Edition, so the content was always around, and I didn't start the side quests until after beating the main game) Once you start playing the dawnguard content the game makes an intimidate shift and you know this is no mere “side quest”; to go fetch a loaf and kill a handful of sewer rats. (But now that's your quest, hop too it Dovahkiin!)
http://steamcommunity.com/sharedfiles/filedetails/?id=517234702

Gameplay: There's no core game changes, except for the vampire and werewolf perks, I cannot imagine being either without these perks, it makes playing as a werewolf or vampire one hundred times more feasible and enjoyable.
Another change to highlight is that with this DLC, you can change your appearance after starting the game, although it's a hefty price to do so.
http://steamcommunity.com/sharedfiles/filedetails/?id=517147101

Achievements: Dawnguard has 10 achievements, almost all of the achievements are easy and straightforward except “Defeat a Legendary Dragon”, Legendary Dragons only spawn after you earn level 70.

Price: Dawnguard is priced at $19.99, you were better off buying The Elder Scrolls V: Skyrim Legendary Edition for $39.99, but if you didn't purchase the DLC, you should wait for a sale. While this content is excellent, for the asking price you could buy a whole new game... butttt the content *might* last you twenty hours, thereby justifying it. (But yeah, just wait for a sale, it's the cost effective thing to do)

Conclusion: Woah Woah Woah, Atratzu? You didn't mention anything about Serana. That's right, because I was saving my rant/love-letter for the conclusion before I recap. Serana is a follower who joins you soon after you start the Dawnguard DLC. Not only is she an awesome fighter, she is she pretty, her voice is calm and quiet, and she has unique things to say based on where she is, summed up... she's my favorite companion and I've brought her with me everywhere in the game.
To quickly recap – Story: Excellent and since it forks, you can replay it a different way next time. Gameplay: Some new changes were added to the core game. Achievements: Straightforward, has one of longest achievement in the game. Price: Fair, but I suggest waiting for a sale, or purchasing the Legendary Edition.
Anyone who enjoys RPG's or Open World games will enjoy the Skyrim series, if you already like Skyrim this DLC is definitely a complimentary addition to the collection.

DLC Reviews
Here's the URL to see my review for The Elder Scrolls V: Skyrim
Here's the URL to see my review for The Elder Scrolls V: Skyrim - Hearthfire
Here's the URL to see my review for The Elder Scrolls V: Skyrim – Dragonborn

------Edits and updates------
Edit/Update April 28, 2015:

[u][b]-Quick Review-[/b][/u] Dawnguard was the first official DLC that Bethesda released in 2012, you gain an additional quest line, cross bows, Serana as a follower, new magic spells, Werewolf and vampire perks, new locations, and more. http://steamcommunity.com/sharedfiles/filedetails/?id=517147037 [u][b]-Detailed breakdown review-[/b][/u] [b]Story:[/b] Long story short; you join up with the Dawnguard to hunt vampires and protect the land, the story splits and you get to make a decision early in the story that changes how the story preceeds. I actually didn't realize I was playing the DLC at first. [i](I purchased the Legendary Edition, so the content was always around, and I didn't start the side quests until after beating the main game)[/i] Once you start playing the dawnguard content the game makes an intimidate shift and you know this is no mere “side quest”; to go fetch a loaf and kill a handful of sewer rats. [i](But now that's your quest, hop too it Dovahkiin!)[/i] http://steamcommunity.com/sharedfiles/filedetails/?id=517234702 [b]Gameplay:[/b] There's no core game changes, except for the vampire and werewolf perks, I cannot imagine being either without these perks, it makes playing as a werewolf or vampire one hundred times more feasible and enjoyable. Another change to highlight is that with this DLC, you can change your appearance after starting the game, although it's a hefty price to do so. http://steamcommunity.com/sharedfiles/filedetails/?id=517147101 [b]Achievements:[/b] Dawnguard has 10 achievements, almost all of the achievements are easy and straightforward except “Defeat a Legendary Dragon”, Legendary Dragons only spawn after you earn level 70. [b]Price:[/b] Dawnguard is priced at $19.99, you were better off buying The Elder Scrolls V: Skyrim Legendary Edition for $39.99, but if you didn't purchase the DLC, you should wait for a sale. While this content is excellent, for the asking price you could buy a whole new game... butttt the content *might* last you twenty hours, thereby justifying it. [i](But yeah, just wait for a sale, it's the cost effective thing to do)[/i] [b]Conclusion:[/b] Woah Woah Woah, Atratzu? You didn't mention anything about Serana. That's right, because I was saving my rant/love-letter for the conclusion before I recap. Serana is a follower who joins you soon after you start the Dawnguard DLC. Not only is she an awesome fighter, she is she pretty, her voice is calm and quiet, and she has unique things to say based on where she is, summed up... she's my favorite companion and I've brought her with me everywhere in the game. To quickly recap – Story: Excellent and since it forks, you can replay it a different way next time. Gameplay: Some new changes were added to the core game. Achievements: Straightforward, has one of longest achievement in the game. Price: Fair, but I suggest waiting for a sale, or purchasing the Legendary Edition. Anyone who enjoys RPG's or Open World games will enjoy the Skyrim series, if you already like Skyrim this DLC is definitely a complimentary addition to the collection. [b]DLC Reviews[/b] Here's the URL to see my review for [url=http://steamcommunity.com/id/atratzu/recommended/72850/]The Elder Scrolls V: Skyrim[/url] Here's the URL to see my review for [url=http://steamcommunity.com/id/atratzu/recommended/220760/]The Elder Scrolls V: Skyrim - Hearthfire[/url] Here's the URL to see my review for [url=http://steamcommunity.com/id/atratzu/recommended/226880/]The Elder Scrolls V: Skyrim – Dragonborn[/url] ------Edits and updates------ Edit/Update April 28, 2015: