Depends on the usage, but I would say that it's no common problem that the letters disappear. It happens, but only in a verly limited number of cases. See, nobody will post something like "hey, I have my iBook now for five years and the letters are in perfect condition", people of course only post when there's a problem.
Get the keyboard, it's awesome. And don't rely on "two button Apple mouse" rumors. If they ever bring out a 2-button mouse, sell yours and get the other one. That said, get a good three-button mouse in the first place.