My advice would be to not use a keyboard protector without some kind of screen protector. The material can rub against the display, and if there's some kind of dirt/debris, it will scratch the screen. There was just a thread on here a few days ago about that.
I've tried a few in the past and I've never used one that is even moderately easy to type on. They're all relatively similar.
The only time I'd recommend using a keyboard protector is if you eat a lot around your computer (my fiancee uses a keyboard protector for this very same reason-- it has saved her MBP from two spills now...) or if you have a lot of pets and hair floating around. Otherwise, it's not worth the hassle.
The keyboard wear is not nearly as big a problem as it used to be. On the previous generation MBPs (before unibody), I could wear off the letters of the keyboard in a year with daily use. The only thing that happens with the current keyboard is that the keys take on a slightly shiny appearance.