They are pretty much the Lada of vacuums. Simple design that rarely changed, common to find in various models, easy to diagnose a problem and repair it, parts are cheap, and they do the job they are supposed to do without fail. No bells and whistles, can suck up virtually anything, and a lot of people have nostalgia for them since they were around for so long.
You could have someone born in the 50's and remember them as a kid, and I myself was born in 1992 and I remember using one as well! It's one of those rare times that a vacuum cleaner model can span numerous generations. Most collectors that just start out, their first few vacuums they get is usually guaranteed to be a Convertible.
The only bad thing I can say about them is their belt is unprotected and directly in the debris path. They can suck up a heck of a lot of things and take all kinds of abuse, but any large solid object picked up will slice right into the belt. But belts are cheap today so it won't hurt your wallet much to change belts as it used to.
Depending on the type of Sanitare model, they used the same design as the Convertibles, and if you figure they use industrial grade belts made of stronger rubber, I can see how they might need some herculean strength to install. If I recall the domestic Hoover Convertible models used a slightly softer and more pliable rubber.
