Having installed 4+ of these things I'll make a couple of comments: one regarding the key stickers, and another about cleaning.
Eric tries very hard to cut partially through the stackup so as to not cut all the way through the glossy backing sheet. Unfortunately, this can result in the cut through the top vinyl sheet being incomplete occasionally.
I have found that it is easier than you would image to delaminate the vinyl top layer from its sticky backing than you would image. If this happens, the key sticker is essentially destroyed.
Therefore, the trick I have adopted is to remove the concentric ring around each key sticker prior to the sticker itself. That way, if there is not a complete cut, it is the disposable ring that gets the damage, not the key sticker. I also have a razor blade or other suitably sharp knife handy to complete any cuts that are partial.
(I was lucky enough that Eric had graciously given me a dud sticker sheet at HHC2011 and I used one of these to replace the one I inadvertently destroyed by pealing the sticker away from its incompletely cut sticky substrate. THANKS Eric!!! :-)
As far as cleaning, I have found that if you use the calculator to any extent prior to installing the overlay, then any residue finger oils will cause the keys to lift eventually. I have this problem with my first batch of overlays (the very first runs Eric made), and to a much lesser extent with the HHC2011 vintage. I would definitely use some kind of solvent to make sure the keyboard is pristine before installing the overlay. As to what the solvent is, someone else might have to pitch in there.
That being said, I had seen a lot of whiny complaints about overlays in the beginning but other than the aforementioned preventative steps, these are absolutely great! Eric has done a marvelous job in producing these things.
Les, I hope you like the calculator. The Triumphant Trio has done a marvelous job on this thing. The more I use it, the more I like it.