A couple of days ago I got a flat tire for the first time in many years.
Here's what I found and the lessons I took away:

I was traveling on a six lane highway when I noticed a loud flapping noise.
At first I thought it was not my car, but possibly a loud muffler from a
vehicle in the next lane.  Then I heard a clunk {whatever had punctured my
tire had become detached} and then silence.

I didn't detect any handling problem but decided I'd better pull over and
check.  I found the left rear was completely flat.  I did not have any major
problem changing to the spare, but I was very happy that my flat happened on a
clear sunny day.  It would have been much more difficult  had it been a dark,
stormy night.

There are two recommendations that I would make:

The first is to expect that a flat might not cause a noticeable change in the
Prius' handling characteristics.  Granted, I was on a straight highway and did
not attempt any major maneuvers.

The second advice is to take a dry run for tire changing before you need it.
It is much easier to determine which tools you will need and where to find
them when you have good daylight.  I also found that the jacking points were
not obvious. There were no diagrams or aids near the jack or spare.   I
actually got out the owners manual to be sure I had positioned the jack
properly.

-Al