I just had my 2018 S100D upgraded to HW3 at the West Palm Beach FL service center, and wanted to share my experience, which was good.
I found out that my car was on the HW3 upgrade list by contacting the service center via e-mail and asking, just before Christmas. It took almost a week to get a response, and they told me that my Model S (but not my wife's Model 3) was on the list and the computer was en route to the service center, and they would contact me to schedule the upgrade when it arrived. I didn't hear anything for a while, so I contacted them in mid-January, and they confirmed that they received the computer, and we scheduled the upgrade for Feb 10 (this past Monday). I could have gotten an earlier appointment in January, but I was traveling so I chose to wait. I also added a few routine maintenance items to the list, because I hadn't had the car serviced since I took delivery in December 2018. Note that I didn't use the mobile app to schedule the appointment, but once they scheduled it, it appeared in the app.
On Monday morning, before I dropped the car off, my service advisor called to let me know they wouldn't have any Tesla loaners available, but they would get a loaner from Enterprise. She asked me if I wanted to reschedule for another time when they would have a Tesla to give me, but I said the Enterprise loaner would be fine. She also told me it would probably take a few days to complete the upgrade, because there were only certain people there who could do that, and there were other cars ahead of me in the HW3 upgrade queue.
I dropped the car off Monday afternoon. This was the first time I had been back to the service center since December 2018, and there was a lot more going on the in the service area than on my last visit. There was a long line of cars in the service lane, and when I pulled into the line, my car was actually blocking the entrance to the front parking lot, but there was nowhere else to go. I waited until I was able to move up and out of the way, then I got out and asked if I was supposed to wait in the car or go inside to check in, and they told me to go inside. I had to wait a little while for my service advisor to free up, but she checked me in quickly. After that I had to wait a while for the Enterprise loaner to be dropped off, but I ended up with a new Mustang convertible, so I didn't mind. While I was waiting, I asked my advisor about the Uber credits, and she told me they do offer those to people who don't want to deal with a loaner.
Once I left the service center, I wasn't able to access the car using the mobile app, as expected. TeslaFi was still able to access the car, however. While they had the car, I received several notifications about software updates (update available, update failed, update complete) and supercharging notifications. They did upgrade the software from 2020.4 to 2020.4.1. I received a notification last night (Wednesday) at 11:45pm saying the service was complete and the car could be picked up during business hours. The WPB service center has a night crew, so they work 24/7 (not sure about weekends). My service advisor texted me this morning to let me know the car was ready, and I told her I'd pick it up this afternoon.
When I arrived to pick up the car, I didn't have to wait to check out, and the car was already parked out front and ready to go. The only thing I had to pay for was replacement of the wiper blade inserts (which I requested). When I got in the car, I checked all the settings, and everything was as I left it (except for the new FSD Visualization Preview setting, which I didn't have before, so I enabled it). I also checked the new Additional Vehicle Details link on the Controls > Software screen (that was added in 2020.4), and it showed the AP computer as "Full self driving computer." I inserted my music and TeslaCam USBs (which I removed when dropping off the car), rebooted for good measure, and off I went.
On the ride home, I noticed lots of new visualizations: traffic lights, trash cans, a traffic cone that was off to the side of the road, a fire hydrant (it might have looked like a traffic cone to the car). One that I wasn't expecting: the car showed turn arrows and HOV diamond markers painted on the roadway, which was cool. While I was at one stop light, I noticed that the display of a left turn arrow on the road behind me danced around a bit, as did the car to my right, but nothing like the dancing cars of earlier builds.
On the highway part of the drive home, I used Autosteer (not NOA), and for a couple of lane changes into/out of the HOV lane, the car alerted me to take over immediately and disabled Autosteer/TACC (I was able to enable it again after the lane change). The car has always had occasional issues crossing the double lane markers that separate HOV lanes, but this seemed more drastic than before. I don't know if it was due to the hardware upgrade, the software upgrade, or something else -- I'll have to see what happens on future drives. Other than that, I didn't notice any difference between how Autosteer worked before and after the upgrade.
All in all, I'm happy with the upgrade experience, and I hope it goes just as well when our Model 3 is due for its upgrade.