I would certainly recommend one - my 770 by now has quite a few scratches, especially along the right hand side (where the scroll bars are) and on the keyboard area, which cause a irritating "rough" stylus feeling. By contrast, the Vikuiti protector on the N800 is practically unscratched, and if it ever should get damaged, it can be replaced.

The overlay it comes with does not really replace a screen protector - that thing is merely intended against in-transport and in-shop damage, and will be wasted within one or two weeks, where serious screen protectors last for years.