We used I-66 under the HO/T rules for the first time this afternoon just after 5:30. The sign on the Roosevelt Bridge listed an $8.75 toll to the Beltway, although the next sign said $7.75. We didn't pay anything, of course, since we put the switch in HOV mode. It felt like about the same amount of traffic there was under the old rules but with a steadier speed—it was pretty much 55–65 mph the whole way, whereas before it usually slowed to about 45 passing underneath that parking structure just east of Glebe Road.
The gantry light flashes a different color when a vehicle with the HOV switch turned on passes underneath—I think HOVs got a red flash and others got white, but I'm not positive. There was a state trooper sitting just after the easternmost gantry, though whether he was monitoring the transponders or something else was unclear.
Flipping the HOV switch to the off position while going down the ramp to the Beltway was no big deal.
All in all, it confirmed my overall position that if you used it as an HOV before, all the hubbub is a tempest in a teapot because nothing changes for you until 2022 when the free ride requirement goes to HOV-3.