"Is this a good accessory to install?  Do many of you use them?  Which ones would you recommend?"

if you are going to do it, an adjustable footbrace seems a must if you have a slider seat.

I never felt a need for a footbrace - I generally brace the knee on my paddle side against the gunnel, and have the other leg stretched out straight - that works for me, and in most cases, i have a pack or something ahead of me I can kindof reach with my extended leg.  In some boats, I can put my toe against the front thwart.  

edit: - its hard to get glue to stick to the vinyl surface of royalex - maybe "G-flex" will work - it would be your best option ?  rubber cement works ok for knee braces or knee pads, but there isn't a lot of "shear" force there like you would have with a footbrace you are pushing hard agianst.  D-ring pads for a royalex hull are normally applied using Vynabond - carefully applied, or you can melt a soft spot in the boat.  

Be sure that any glue in footbrace you buy will have glue that will work on a royalex boat - i.e., if you want to buy a Bell footbrace, call them first and tell them you want it for a royalex boat, and ask if the one you are thinking of will work.  I'm guessing they will recommend a riveted approach.