For a number of reasons, I'd like to install a rain barrel. I've
started to look at them online and I'd love some insight from any of
you that have one.

Well, the bigger the better (the more ambitious versions would have a
scavenged RV water tank, a series of rain barrels chained together, an
underground tank, etc). Of course there is nothing wrong with working
with the space you have, testing the waters, etc.

Our 65 gallon rain barrels fill up with any moderate rain (by east
coast standards - maybe a quarter inch to an inch, not that I've tried
measuring closely). We fill up our watering cans with them, and
depending on how much watering we are doing, a full barrel might last
us a week or a few (which is roughly the time period between rain
here).

You'll want some kind of mosquito screen (most commercial barrels have
them).

Ours has a spigot to get the water out, at the bottom of the barrel.
That means putting the barrel on a cinder block or some other
platform, so there is room for the watering can under the spigot.

It can be helpful to have a guage to see how full it is (although we
don't, and just make do by tapping it, guessing, and/or unscrewing the
top).

Any other questions? I'm not sure other parts to talk about.