I have a Canon G5.
I use the LCD only when I'm shooting at waist level or above my head, or at a right angle, and the viewfinder at all other times, except of course when it's totally useless, and that's when I've attached an adaptor and a filter or an accessory telephoto lens. At those times I wish I had an accessory viewfinder.
I suspect an accessory viewfinder will affect focusing and composition, and I'll likely have to do some parallex correction, but I'm sure I can live with that. The only problem I have is having no idea whether it's really feasible, and if so, which one I should buy.