My large company uses TWiki successfully with over 1000 users and thousands of pages. I'm a user myself and can review TWiki from the user's perspective; I haven't administered it.
TWiki has a simple syntax that is very easy to learn by non-technical staff, and also has very powerful expressions that make it possible to embed complex searches right in wiki pages. Its layout is to a degree customizable even by users via "web"-wide variables. It's not VERY fast, but we do have thousands of pages.