This DLC is not only very good if you like Vikings, it is also a very important and positive change from Paradox's DLC policy of old. Let me explain. This is a Viking DLC, there are new mechanics and events for people of Norse religion and/or culture, and there are also a lot of cosmetic features like the new Norse UI, music(absolute bangers by the way, but don't expect Viking metal, it's much more folk and atmospheric), clothing, 3D models, essentially anything a Viking lover would wish for. Now, this seems very standard for a DLC, but not for a Paradox DLC, since if this were an old Paradox DLC this would also have some features available not only to the Vikings, but to everyone, forcing people into buying the DLC even if they didn't care about Vikings.

One example would be Jade Dragon. That is a DLC for CK2 that adds off-map China mechanics into the game. What, don't care about China? Too bad, the DLC also has rally points and new Casus Bellis available to everyone, so if you want them you must buy it. This sort of shady business practice was commonplace in the old DLC policy of Paradox and it's even present in Imperator: Rome, the most recent Paradox game after CK3, so I was
positively surprised to see that this Flavour Pack doesn't have anything like that at all. I'd go as far as to say this is a prime example of how a DLC(flavour packs like this at least) should be made. Of course, we'll just have to see if they keep this up.

Oh, and most of the negative reviews are from people salty about DLC for no particular reason. This did come out with a very good free patch which brought new absolutely free features like winter, poetry and dueling. Even some things that Paradox could've rightfully made part of the Northern Lords (like new religious icons and Viking Coats of Arms and mottos) were also released for free, and since I already explained how this Flavour Pack is much more customer-friendly than what Paradox used to make I'm not sure what the haters are even mad about. Perhaps they'll explain their problem in the comment section, but most likely I'm just going to get called a shill ¯\_(ツ)_/¯. One criticism people expressed is that all people became bald in save games from before the current patch, but that's a problem with the free update, not the DLC and the problem goes away after a few generations or if you start a new game anyway. The real sad thing is that old DNA strings also feature bald characters, so now I have a txt file with lots of bald characters' DNA, which is a bit annoying but again: this has nothing to do with the DLC, if this mild annoyance somehow completely ruined your game then your review belongs under the main game, not here. Other than that I'm not sure why anyone wouldn't recommend this(haven't even encountered any bugs), since even the price is very reasonable.

TL;DR in 3 points
1. Get this if you like Vikings, you'll love it.
2. Unlike old Paradox DLC there are no shady tactics used in order to sell this to as much people as possible.
3. Negative reviews are mostly people being salty about this DLC for no reason other than that it's a DLC. Don't worry about bald people.