It depends
I used one for a while years ago back when I was grinding every day, and it definitely helped me, but I went in with some very specific goals around improving my STT game. I found a site that had a good coach on that topic and yes, it helped.
I found going in with a specific goal was key, that allowed me to find a site that was a good fit and kept me focused on what I was working on. I was also at a level where the free resources (like discussing
hands on here, reading general books, etc) had gotten me to a certain level but I wanted to move beyond that.
If your goals are more general, like "just get better at poker", or if you're not at that "taking the next step" level yet, you might be better off focusing on the free resources instead. Post and discuss more hands on here in the hand analysis forum for starters.