Got my Rift working with AC and iRacing last night. Unfortunately, I came home from work with a pretty bad headache which isn't good news for a first attempt at sim racing VR. Jumped into a Miata first in AC and got queasy just a couple corners in so I stopped. It didn't take me long to get over it but I'm blaming most of it on the headache; the sense of immersion is unlike anything else though. I'm anxious to try it out again today and see if it really was the headache or it's really going to take me a long time to get adjusted to it.
I tried out Eve Valkyrie late last night for ~20 mins and it didn't bother me at all.
EDIT: First go at it this morning; fired up rFactor 2 (had to set up Steam VR first). Tried the USF2000 at NOLA, VERY impressive! Much less of a queasy feeling today too. Will say I did 5 or so laps there then switched to the Formula Renault at Silverstone for a couple laps. Started to get a little queasy after that one so I've taken an hour or so break. Going to have to ease myself in as excited as I am about it.
Keep the impressions coming - my Rift doesn't arrive until the middle-end of next week.