I did one with Edel. It was free, the local clubfitter was doing a promo for Edel, and I was fast to the e-mail and got one of the spots. I'd consider myself a better than average putter, but my power fade swing has put my body and eyes into a permanent open alignment. So 90% of my putt misses are left.
The bad thing was, it was done in the clubfitters shop, so it was inside on their practice green which had about a 15 foot max putt. Still, that should be enough to find alignment issues.
The fitting was pretty interesting. They test out shaft lengths to see what feels best, and then swap out head styles. They mount a laser on the clubhead to see where you are actually aiming. I wasn't as far off as I expected. In all, it was about 45 minutes of try this, try that.
Once he had me dialed in and complete, he put 10 balls on the ground at about 12 feet from the hole. I made 6, with 2 lip-outs. Pretty pleased with that. Then he told me to grab my old putter and give it a try. I made the first 8 putts. No sale for me that day.
IMO, fitting for a putter is the least valuable fitting. Lessons are much more valuable there. But then again, I've done so much buying and selling of putters, I pretty well know what length and club head style works best for me.