IMO yes. Although reception to it has been rather mixed so you might not agree. It's also an 'optional' DLC that doesn't really affect anything if you don't play as a nomad.
Horse Lords adds the Nomadic government type. Nomads are very powerful on the offensive since they rely on retinue-like Horde troops which can reinforce and are easily customisable. They also have lots of offensive CBs, including a single-county conquest CB, a duchy-level conquest CB, an interesting Kingdom-level Invasion CB that depends on population levels and a possibly-overpowered subjugation CB that can destroy entire non-nomadic Empires.
In exchange they are supposed to be very unstable since they are forced to consolidate their realms under very large powerful vassals called clans, which have a unique relations system. It's not really possible to go North Korea Mode as a horde, even temporarily. Horde lands have no settlements (except the capital encampment) so they can be occupied very easily without a siege. As with tribal counties, feudal kingdoms can build forts in occupied horde lands in order to keep them from being re-captured easily and help with supplies. Because provinces change hands so easily, the real strength of the Hordes comes from their, well, Hordes. Battles with Nomads are usually very decisive. Nomads also have a rather unpredictable succession system in which the son with the most prestige inherits, and the clan chiefs can and will attempt to seize control by force if they're dissatisfied for any reason.
The problems with Horse Lords mainly stem from the fact that Hordes are *not* actually very unstable, despite the developers' intentions. Civil wars are frequent but hordes rarely actually break up. In the beginning of the game the small Central Asian hordes (like the Turks, the Kipchaks and the Karluks) will quickly be conquered and destroyed by their more powerful neighbours in Mongolia and the Pontic steppes, Russia will fall to the Khazars and eventually the whole northeast quarter of the map will end up dominated by one or two enormous nomadic superpowers. The game appears to be designed to make settled peoples hate nomad rulers (in order to further destabilise the Hordes) but in a bizarre decision the developers decided to make tyranny basically inconsequential for them, meaning that nomads can just revoke every single settled title without consequence. This might be different now with the altered council (I haven't been a horde since I installed Conclave), but in my experience a nomadic empire can just rule by terror and crush everything beneath their horses' hooves. North Korea Mode might be impossible since you need to have clan lords in your empty steppe provinces, but settled provinces are fair game. It's possible to hold the entire Byzantine Empire by yourself as long as you allow the clans a few scraps of land in Hungary.
That being said, this is a genuinely fun expansion most of the time. Nomads exist off-map, they cannot be destroyed except by killing all their soldiers, so if you find yourself annexed by the Uighurs you can declare an invasion and migrate off to some distant country for conquest, provided of course that they didn't kill all your soldiers. Raiding is brilliant fun of course (as with pagans) and from a role-playing perspective the range of possibilities for interesting conquests is amazing. Maybe a band of Alan mercenaries will leave the steppe and journey to Brittany or Spain and set up a Catholic kingdom of their own? Maybe the Karluks will unify the steppes into a great empire, only to be crushed and forced into India? Maybe Turks will aid a Zoroastrian rebellion/restoration and destroy the Abbasids? One of the most fun play-throughs introduced by the Old Gods expansion was as Hastein, going around the world and setting up some exotic new Norse kingdom, and Horse Lords really doubles down on that sort of great migration experience. Once you actually start conquering the world you become an unstoppable superpower and the game turns rather easy, but isn't that unfortunately the case with the rest of CK2?