It's out now, at least on pc.
So I'll be giving my first impressions from storming through the thing with my good friend Dreams-of-Warm-Boots
Dear journal, have just arrived at Solstheim. Warm boots are cold now, makes me sad. Volcano is pretty though and tapped tentacles with cute netches. Also found nice sword, better than hammer. Me is happy now. That's nice for you, Dreams. That's nice.
On the whole, I like what I've seen of it thus far. Dragonborn's location is made with the Morrowind nostalgiast in mind. The Dlc is simply chockful of references to the Bloodmoon expansion, both in places, in characters, lore and even some quests. But don't think that Solstheim is a carbon copy of the island from Bloodmoon. It isn't and it shouldn't be. The general shape is the same however and there are the aforementioned locations, if often reimagined and quite possibly moved and lets not forget the addition of new places to go. The reimagining mostly draws on existing assets from Skyrim so barrows now look like Skyrims instead of the square blocks of Bloodmoon.
Solstheim is mainly populated by Dunmer now, Morrowind Dunmer. Though they don't have the awesome gravelly voice back, that means you too can be called a fetcher once more! And as Dreams wrote in his journal, they imported netches and other animals that call back to Vvardenfell. And btw, I love that they added Red Mountain to the skybox.
On the new front, there is more lore and books regarding the dragons for the sake of connecting the expansion with the main game. I've also explored a bit of a new type of dungeon. And while I'll refrain from saying too much cause of spoilers, I have to say it is visually a very interesting departure from the old dungeons and ruins. Linear as always but very...creepy and with a nice sense of scale.
There are also more weapons and armours and more shouts for the interested adventurer. But I haven't really looked into that aspect much up to this point. Dreams is very happy with his furs, they keep him warm.
The quests I've triggered and finished so far aren't bad either. I've met some nice characters in it. The worst of the lot is the intro that sends you off to Solstheim in the first place, however. It's a bit abrupt and very blunt in setting up the main quest. Pretty much the same way the Tribunal expansion was introduced for guess what, Morrowind. But ah well. New island to explore, new stuff to loot. Live is good.
In short, the expansion may or may not be worth the price to you, and morrowind vets may be a bit angry at some of the things they did after they were done plundering the countryside. but it is definitely a lot bigger than say Hearthfire and I'm pretty sure it also beats Dawnguard in content.