We bought a service loaner S100D back in December. The process was smooth. I did NOT find and reserve the vehicle online (read horror stories on here about people being bounced around between sales people, failing to get timely responses, etc). I called my "local" store (Kansas City), and one of the reps there (Harmony) took great care of us. She found a few cars that matched our specs and sent them to us. We went back and forth a few times and she was always prompt with her responses. We picked one and she took our credit card for the deposit over the phone. The car was in Indianapolis and they wanted $1k to ship it to KC or for the same price they could deliver it to our house in OK. So we arranged to have the car shipped to our house.
There was self-created drama as there was about a week where communication stopped while the car was being shipped from Indy to KC. I started getting nervous about getting the car before the end of the year (for the tax credit). But the car arrived in KC and was promptly shipped down to us with plenty of time to spare.
We traded in our P85. The trade-in offer was on the low side but was painless. Tesla delivered the new car and took the old one. New car was flawless save for the yellow screen disease but I had read about that here. I'm a bit miffed that I didn't read here about the lower charge rate from the new UMC. I was going to just keep our old one but the new one looked prettier, had a nicer case, etc. So I sent the old one with the old car. Then I learn that they pinned the new ones to 32A. Argh.
The car had a Missouri temp plate on it and Tesla FedEx'd the paperwork to register and to transfer ownership of the old car. Everything was done before they shipped from KC. You are able to upload the docs via your Tesla account as well as pay for everything right there. In all a great experience.
My advice is to figure out your closest store and work with a rep there vs going through the website where you have a pool of reps. Good luck!