Really easy.
The downsides if you leave them as easy to put on and take off you keep the two metal brackets under the brake calipers (Mudguards do not protect the brake calipers) They do not extend far near the seat tube (I had to cut the rear seat tube part to stop it hitting the seat tube over bumps) So the Front Mech still gets spray through puddles. Only deep puddles at speed get your feet wet.
The upsides your feet dont get splashed in normal wet roads, back side stays completely dry, fast to take off and put back on again, maybe takes me 5 minutes to re-install them and less to remove them off the bike. The are easy to adjust so they do not rub, they fit in to the quick realese skewer and have a quick release attachment to the metal brackets under the caliper. Compare to crud catchers they are genius, I am only aware of them over bumpy roads at a reasonable speed and that is no more than I am aware of ones I have on a bike with mudguard eyelets.
It really is as simple as this video, I have left those metal brackets in place, when the weather looks good I whip them off and stick them back on if it looks like rain, if it was not that easy I wouldn't bother.
View: http://www.youtube.com/watch?v=DiBkUT1Kf6E