I'm just curious, because I love wordpress.
visit for demos
The Next Web is a good example of large-scale, high(ish) traffic MU site. Well implemented with a good design.
EDIT: As agreed upon in the comments (and quite evident from the site itself), TNW has switched from WordPress Multisite to a single site setup.
I think, to be blunt our own site is working very well and looks pretty good. The url is http://bloggersdelight.dk - Currently arround 10.000 blogs and around 15mill pageview / month.
Setup is scaled and balanced using several servers.