Has anyone done it? Was it worth the cost?

Why on a portable generator? I'm sure you have a reason(s), but seems a lot of negatives from my view of propane.
 I can see it on one for the house where it would be plumbed into a bulk tank.

I'm thinking it's because of how bad the gas is these days..and leaving it in the tank or even storing it.

I'm thinking it's because of how bad the gas is these days..and leaving it in the tank or even storing it.

This ^^^^. All we get is alky contaminated fuel. Even with proper treatment for storage it doesn't last. I might not use the gen set for a couple of years and I thought it might be easier to hook up the bbq tank than chasing fuel in the middle of an emergency. The duel fuel option would also be a plus.

I'm thinking it's because of how bad the gas is these days..and leaving it in the tank or even storing it.

Correct..... With propane in the bottle it never goes bad and is really easy to handle as well store 20 pound or 100 pound bottles rather safely.... Slight loss in power over gasoline.

Has anyone done it? Was it worth the cost?

Regardless if portable or built-in my plan if I decide to install one where we live is to go with Propane... As it now is I run our fireplace on 100 pound bottles; while a bit impractical for a permanent fixture to the house such as a fireplace it is my only easy low cost option.

I didn't do a conversion, but my power ice auger for ice fishing is a propane.  Jiffy Pro 4.  Starts much easier than any other I have used.  Cuts hundreds of holes on a 1lb can, and has plenty of power.  Only down side is it can spill oil if tilted the wrong direction. 

Only once did I have an issue in the cold starting my auger.  -38F (air temp) it would start, but would run short of power as the 1lb can would not "boil" fast enough to meet demand.  Just changed the tank to a warm one & drilled all the holes I needed. 

I would do some checking into how much fuel per hour the generator in question would use under load. During a power outage a generator can go through a fair amount of gas (propane or gasoline). Might need a good stockpile of propane cylinders if not hooked up to the bulk tank. I don't think 20# of propane will last as long as 5 gallon of gasoline running under load.
http://www.energyalmanac.ca.gov/transportation/gge.html
Chart shows 15 gallons of propane is equal energy to 11.16  gallons of gasoline.
4.2 pounds per gallon so a full 20# tank is about 4.75 gallons which have the gasoline equivalent of about 3.5 gallons.
https://www.propanecarbs.com/small_engines.html
Conversion isn't too complicated.

I think I'll stick with gasoline. Easier for me to store and transport enough of it. I start the generator(s) at least once a year. Run them a while and change the fuel. So far no issues. Had a diesel generator (actually two different ones, one was Chinese and the other was a sweet twin cylinder Duetz diesel) for a while, but chance of getting those started if the outage was -30* winter time was slim to none.

I just ran mine for about 1/2 hour this past weekend. I like to do that about 3-4 time a year...if I remember it. I plug the house in & put it under load, too.