There are sensations during the event that stem from two sources. 1) The rapid descent itself, which is described in the above posts. Expect partial g force while the aircraft initiates descent, a gradually increasing nose down pitch to a steeper angle than most would be familiar with, and (2) the reason for the rapid descent, which in this case is a loss of cabin pressure. (Not experienced this myself, but spoken with medics and pilots who have practiced it in the pressure chamber.) This happens very fast in an explosive decompression. There would be sound, a big gust of air, taking lightweight or not- secured items with it, abrupt cooling of the air inside the aircraft (which causes the white mist often seen in these events, as the water vapour in the air condenses out), ear-popping, and flatus, as the body's internal pressure tries to equalize.
Not seen this, but I'd imagine it would be all over in a few seconds, then you'd expect to feel the nose drop, maybe combined with a fairly steep bank angle on some types. If (as it most usually would) the nose stops dropping, and the dive stabilizes, that's a big clue that the aircraft is under control, and the dive planned.
I've been in an airliner where this was simulated, and simulated the motion numerous times in light aircraft. It can be a somewhat revolting feeling until one is used to it, and, especially combined with the noise, wind, white mist etc, almost certainly gives rise to the reports of passengers thinking they are in a death plunge.
In the highly unlikely event you are ever in one of these, don't bother trying to grab the mask until the wind dies down and it stops jiggling around. Put it on ASAP, pull the tube (as directed,) breathe deeply. Then, once you've noticed the dive stabilizing, and noise permitting, you can reassure those near to you (if they need it) that the dive is a controlled one.
Personally, I think the surprise of being in one of these would make one forget all this, unless one repeatedly trained for it.