Huge advantage to sailing performance... with a higher cost (to buy) to go with it. Depending on the type, side effects can include reduced effectiveness in reverse, vibration, occasional failure to fully open for some.
Feathering props are generally more costly than folders, but perform better all around, esp in reverse. They can range from Kiwi props at around $1600 to Max props at $3K+.
Geared folders are better because they prevent the 'shake-your-boat-apart-single-blade-opening" syndrome.
Overall they are worth the expense unless you REALLY don't care about sailing speeds. For us even though we don't race we do like to keep the boat moving at the optimum speed for the conditions. A fixed prop compromises the ability to do that, esp in light air.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"