I've had 2 in the past, and also worked for a team that trained coaches and sought volunteers for the trainees' case studies so turned over quite a lot of 'clients'. Having seen the results that one of the trainees got (who was also training in counselling, separately), I wanted to be (and was) a guinea pig for them.
There are lots of different styles. For me, I did not find useful at all the types who base things a lot on certain sports psychology techniques (such as having a discreet finger sign as an 'anchor' for confidence, power poses, a deep belief in affirmations). Just not something I can swallow.
I was also deeply sceptical about psychometrics but having done a few now, with no assessment resting on them, I am somewhat won round but would recommend something not general, which is most of the better-known ones (e.g. looking at leadership, strengths and strengths 'in overdrive' and how they then start to get in your own way, or similar. That was useful and quite affirming).
The important thing is that you do the work through it all. The coach will be a 'champion' for you and try to show you where you're viewing things in a skewed way. I would recommend that in an early discussion you also ask them to help hold you accountable for things that you want or plan to do (however you want to do that), and see how they react to this. They do help you do tricky things like ask for feedback from people who do know you, which is awkward.
Paying - yes, but not over the odds. Perhaps for something like 4 sessions, well spaced, to begin with? Trainee coaches, if there are any needing people to sit in the coachee chair as practice subjects, will be variable but mostly pretty good on accredited programmes.