A Rivian R1T owner that took delivery not long ago has been giving some feedback on what the experience is like.
https://www.reddit.com/r/Rivian/comments/sy3dw3 Here's some that stood out to me:

Q: What are some examples of the gripes?
A:
UI is sometimes slow to load when you first get in the truck. Also, navigation is just okay. Its fine for navigating to and from most places unless you want to optimize for traffic. Also, trip/charge planning is better on your phone. It will be cool to see how these features improve.

Q: How was range anxiety, charging, performance
A: So I came from the South Bay (area). I've done multiple Tahoe trips but this was my first mammoth trip.

Range anxiety wasn't bad. I knew I could make it to each charger. I stopped in Rocklin before going up 80 towards Tahoe. I stopped overnight in Truckee area charged at the EA station there before heading out. Next stop was Bridgeport. Then mammoth. On the way back, I hit Bridgeport again and another stop in Folsom Area. So stopped twice from mammoth area to San Jose. Each stop, I probably charged for 20-30min each time. I usually never go above 85% on longer trips. I probably could have gone on from 395 to 88 past Kirkwood and charged in Stockton area instead of down 50 to Folsom to save a bit of drive time. I'll probably do that next time.

As far charge rates, I believe this was right before an OTA update. I am currently getting about 180-190kWh from 20% to 65% and then it drops after that.

Performance was great! Highway assist is awesome and its gotten better from each update. Makes long drives like this super easy. Ride is super comfortable. I didn't hit any snow on the roads (just off-road) so can't comment on that. I love being able to change the suspension settings from soft to stiff for more curvy roads.

Overall, the truck is awesome! Mechanically, everything works great. There are some small gripes here and there but I believe these should all be ironed out with OTA's.

Q: What all does the Highway Assist do? I’m only familiar with Tesla Autopilot and have been curious if others match their level of automation.
A:
Basically, steers and brakes for you on major highways. You need to keep your hands on the wheel, but can take your hands off for ~15 seconds at a time without the system beeping at you. You set your speed and distance to maintain between the car in front of you and sit back and relax. Currently, there is no lane change feature.