I haven't but a very good friend of mine did a couple months ago and so did my nephew last year and both had good experiences (I believe they both used Carvana).
When I bought my last Jeep, I considered buying used and looked on Carvana and Vroom (looked at Gladiators, Grand Cherokees, and a few other vehicles) because there was no inventory locally and noticed prices were all over the place, especially for the used Audi and Volvos I was interested in. Ultimately, I ended up buying new from a local dealership but I'd still consider Carvana or Vroom for a used vehicle... It just seems like there are infinitely more choices to choose from. I would sort of like to get an older Land Rover Discovery or LR3 for something I could eventually turn into a trail rig if I found the right deal but I can never seem to find them on those particular online services; maybe those models are just too old for them to deal with.