Before I ceased working in the field, while we knew of NIBP machines, we were never allowed to have one unless we were working on the CCT bus, then we got to "play" with our Propaq Encore. Nice little monitor...
What that meant is that all of the vital signs we took were manually obtained when we weren't on CCT duty, and we were generally required to get 3 sets on every run (with few exceptions. What this led to was me doing somewhere in the neighborhood of 20k sets of vital signs (conservatively) during that time.
I trust my ears more than I trust the machine when I'm bouncing down the road. While that does tie me up for a while, it's not that long and I get some decent info that I trust. I do trust the machines when they're regularly maintained and they're reasonably accurate when stationary. Also, while I do kind of care about the values presented from moment to moment, I am far more concerned about trends, up, down, flat, widening or narrowing...