If your shop/work area can accommodate the 100 gallon tank, and you're in it for the long haul, then it is certainly worth checking into.
Several years ago I lived on a small farm. All heat for the house (stove, water heater, furnace) was supplied with a 500 gallon tank. Disregarding lifestyles, home efficiency, "rough winters", etc., we averaged about one-and-a-half tanks annually.
We all have different applications and conditions, but what has been said by the others should hold true- a 100 gallons of propane should last you quite some time.
If you can get a rough idea of how long a little 20# (5 gallon) tank will last you, simply multiply by 19 or 20 for rough estimate on the large tank.
For me, the major advantage of a large tank is not the longer interval between refills, but the ability to handle freeze-up.
Like most of us, I use the 20# cylinders.
But if I'm really puttin' out some serious heat (large venturi burner/high pressures), the small tanks will ice to the point that pressure cannot be maintained.
There are a few solutions to the icing problem.
Some people place the tank in warm water; I have done that before and it's so much of an inconvenience that I don't care to repeat the process.
Another option is to connect more than one tank, and this is what I currently do. Two or more tanks (I use three) connected via a manifold will alleviate the icing problem, but all tanks will empty at the same rate. More connections to mess with, and more chance for something to go wrong. My largest complaint with the multiple small tank approach is the floor space used.
And then of course you have the option of drawing from one larger tank.
But for me (and I suspect most of us), the small tanks are about convenience. We want the ability to place the forge and the tank in different locations so as not to have a mile of gas hose laying across the floor. Plus, it is more convenient to refill the small tanks. Most areas have more serious regulations pertaining to transportation of tanks much larger than 20#; I think 40# is the "limit".
Of course, by the time we get to 100 gallon tanks most of us are considering delivery service from the gas company.:3:
(An empty 100 gal tank should weigh close to 300#, and it will hold over 400# of propane. Somewhere around 700# total +/- when full.)
My problem is that my location doesn't have easy access for a fuel truck, and I don't want to run lines underground from another area. If I went to that trouble, I would get a 500 gallon tank and use it for everything.
If you have the means to facilitate refills, and your shop area can accommodate it, a large tank is a good choice.
With the large tanks the best thing to do is locate the tank in a separate area; most folks build a separate "room" on the outside of the shop and run hard lines around the walls inside. Various hookup locations around the shop (with shutoff valves at each point).
Weigh the options, and decide what you need.
As always when dealing with used tanks, make sure the tank is safe to use before plunking down the cash. $100 for a tank that size is a great deal if it's in usable condition.