I did adult PC for a few years. PC as a kid would have been one of my ultimate horse dreams (inspired by the Cammie books), but no way it was happening way back when for a horseless youth of limited means. It might happen now for a kid like I was back then, and that is a good thing about some of the ways PC has changed.
My experience as an adult was mixed but probably largely colored by being part of a small club that struggled for most of its existence to have enough active members to be viable. Because we were a small club, it was not as much of a bargain financially - we used more of a “pay as you go” model. Why that club was formed (when it never should have been) is a subject for a whole 'nother post - if I had joined the other club local to me, I think my experience would have been quite different.
I enjoyed the group lessons and exposure to instructors I would not have otherwise ridden with. I went to camp a couple of years (it was hot, hot, hot). In order to keep the club going, I took on the role of DC. In my second year, we were actually going to have a club team (that included me!) for the regional dressage rally - and then the pandemic hit. Over the course of the pandemic, we lost a couple of members, and potential members kept choosing the other local club over us. Eventually we ended up transitioning our name, banner, and bank account to a new club in a different part of the region.
Even though I was looking forward to the dressage rally back in 2020, I did NOT find the rally format as a competitive outlet for an adult very appealing. For one thing, I would have paid about the same cost for the rally as for going to a local dressage schooling show - and then add all the overhead of inspections and HM… um, no thanks (though I can certainly see how as a youth being on a team in the barns without adults telling you what to do would be great).
I really like that PC sees itself as an educational organization with a primary mission of teaching horsemanship and correct fundamentals. I really like that progression through the ratings is standards based. However, as with many organizations that are volunteer based, the experience can be quite variable depending on where you are and the quality of the programs in your area. As with many organizations, there are usually a few dedicated volunteers who do the bulk of the heavy lifting - whether that is at club or region level.
I personally don’t like the addition of all the disciplines that aren’t traditional to Pony Club - I think in trying to include “all the things” you water down your core. There is a lot of competition in our area for the riders/equestrians who are part of the PC target demographic (PC, 4-H, IEA/IDA, high school equestrian) and although a few people try, you really can’t do them all.