It was... Alright, I guess. I'm an outspoken critic of the overplayed vampire bullshit in Skyrim, so I'm probably biased, but here are my thoughts...
This playthrough was with the Dawnguard. I'll probably post a follow-up when I complete it as a Vampire.
As usual, Bethesda doesn't disappoint as far as content goes... The new areas and maps are gigantic and gorgeous. Sadly, this is also my biggest gripe by far. Dawnguard took me ~7 hours to do one run-through (one faction, not both), and the vast... VAST majority of it was little more than traipsing around the environment trying to get to the next objective marker. There's little to no story or content inbetween the objectives... Just long, long.... LONG walks.
Sadly, I died due to my own stupidity in the Soul Cairn and was forced to start back at the beginning. Man, that was when I first realized just how long and tedious the walks were. It took me like ~10 mins of pure walking JUST to get to an objective where something would happen. During that walk, there was absolutely nothing to pique my interest at all other than 1 random, annoying forced battle with shitty skeletons.
As far as the story is concerned... Meh. Storytelling is never a Bethesda / TES strong point, so I guess I really can't complain - it's just more of the same. It's not particularly gripping. The motivations of Serena never make any sense, which is sort of bad since she's basically the point of the entire DLC.
One thing that was surprising / disappointing to me was the lack of outright Dawngaurd vs. Vampire fights. There's really only one direct confrontation at the very end of the DLC. Prior to that, it's just you and Serena killing 3-4 random Vampire spawns. In fact, you kill more Falmer and Frostbite spiders than anything; there's a really long, extended snow-elf / falmer sequence that seems really out of place yet constitutes a huge portion of the content.
Characters on the Dawnguard side are down-right bad. The only person who I even remember is Isran, the leader, and he's the generic gruff-voiced, stern Nord that we've come to expect. What's wrong here is that during the quest you have EXTREMELY little interaction with any character other than Serena. You talk to some nervous recruit at the beginning, get a single quest from Isran, and then the rest of the quest line is 100% adventuring with Serena. And you can't even marry her.
Armored Trolls are bad-ass looking and sound great. There's nothing more hilarious and epic than seeing one growl like Chewbacca and run into a group of vampires that are hurling spells every which way. Unfortunately, though, they're pretty damned useless. They can't even keep up with you when you're walking and they're downright bad in combat (if they manage to get there before you win or die).
I am pretty pissed because I lost Orchendor near the beginning of the quest line, so I'm going to have to re-do everything to get him back. I guess it does give me an excuse to play on the vampire side.
All in all, Dawnguard was a decent expansion. I don't regret buying it because it gave me more of the game that I already loved. The thing is, though, that's all the Dawnguard side of the story felt like it was intended to be - a drawn out excuse to walk through a few more caves and get in a few fights with Falmers.
If you're only going to play it once as the Dawnguard it's not worth $20 unless it's your thing. I paid $10 when it was on sale and that's probably what it's worth to me. Nothing ground-breaking. I hope the Vampire side will be more interesting because I can be a Vampire Lord at the very least.