We have had a two blade Varifold/Brunton folding propeller on our E30+ for the past 11 years, and would recommend it highly. It used to be that the blades on a folding prop would not always open, or open together, but now they are geared, so they work together to open, so I have never had that problem. It is important to keep barnacles and growth out of the gear area, and on the propeller as a whole, as it has moving parts which will not move as freely (and fold under sail) if it is fouled with stuff.
I figure that it increases my boat speed by about a half knot compared with a regular non-folding propeller. Also, when folded, it doesn't pick up weeds as much as an open propeller would.
I have read reviews on them, and they now indicate that this (and similar) folding propellers work just as effectively in powering the boat. My boat doesn't do well at first in reverse due to substantial prop walk, but once underway in reverse, it's fine. I don't know if it would be any different with a non-folding regular propeller. While I have a two-bladed one, they do make them with three folding blades as well. I don't know how much difference that would make in forward or reverse.
In summary, although they aren't cheap, I would recommend it.
Frank