" Will thermal curtains or drapes help to reduce the heating and
cooling
enough to make buying and hanging them worth the effort? "

The way I would approach this is if I needed new curtains anyway, then
I would take energy issues into account when I bought new ones. The
incremental cost may be worth it over time. However, if I had
curtains that were fine, I wouldn't buy new ones expecting to come out
ahead in energy savings, as I doubt that will happen. You can get more
mileage out of what you have by making sure they are closed at night
and open any on the sunny side during day.

Also, as others have suggested, I'd look at other things that could
have bigger impact and for less money. There are websites that will
give you a list of things to look at. For most houses, fixing simple
things like weather stripping around doors, making sure the water
heater isn't set higher than necessary, installing an auto set back
thermostat, replacing regular bulbs with CFI where practical, etc, will
have a bigger cost/benefit than replacing drapes.