It is more common for modern mountainbikers to use adjustable, or dropper posts, rather than suspension posts, both on hardtail and full suspension bikes.
The difference is that a dropper post's length can be dynamically varied with a lever press, but without pressing it there is no spring action what so ever. This way, the rider's center of gravity can be lowered on rough and steep descents, but pedaling efficiency can be returned back whenever needed at the post's full extension.
In contrast, a suspension post has a fixed length and saddle position oscillating around that length with help of a spring of some sort. It is typically used for comfort riding or touring long distances on rough roads.
No modern high-grade modern mountain bike known to me comes with a suspension post, but a lot come with a dropper post.
Is it worth the money
It is only for you to decide.
does it improve the ride quality?
Having a bit of something to smooth road chatter is always welcome. Think about the trade-offs however: higher weight, higher cost, need for maintenance, and occasional return spring action smashing the saddle into your crotch (a spring saves kinetic energy only to give it back later). For mountain biking, where road chatter is not an issue, I would always prefer a dropper or even a classic rigid seat post to a suspension post. For a 100+km gravel road ride, I would go with a suspension post.
I want also to add that a suspension post is likely redundant on a full suspension MTB because there already is a spring at the rear of the bike that should provide similar, if not that pronounced, effect. For hardtails, modern frame designs attempt to build limited amount of flex into rear parts of frames (seatstays) to fulfill the same purpose of filtering out the road chatter getting to the saddle.