Borrows heavily from classic vampire stories and a lot of their cliches.
Level design is probably the best of all Skyrim VISUALLY. In gameplay, however, is a bit of mess, and characters that focus on stealth will be forced to take a warrior / mage approach in half, if not more of the encounters in this DLC.
Some parts of the story feel quite rushed, so by the end you become the big bad boss and doesn't feel like you've earned it at all. The story progressing so quickly also meant that it was difficult to get invested in what was going on.
The elder scrolls themselves lost a lot of their magic and mistery, by suddenly having 4 at the end of the expansion.
Also, might not be related to the DLC as much as the game itself but Serana, despite having great stats and abilities, proved to be by far the most incompetent follower I've ever had, somehow being absent in countless battles, having a particular talent for getting stuck in all sort of places and falling behind when moving from point A to B. She was better at preventing me from completing my objectives than all of the enemies I faced throughout the expansion put together.
Seing the Snow Elves was great, even if the circumstances of their existence are very similar to Yagrum Bagarn. Auriel's Bow went from being a generic weapon to an amazing piece of equipment; the mechanics surrounding the manipulation of the sun are very well done.
Overall, I'd say that this is a mediocre DLC with flat, boring characters and a generic story, and would suggest most people to simply skip it. Wouldn't say the same about Dragonborn, which I hold in a much higher regard than this one.