In our case, it isn't a large system. Our node production environment serves up a smallish set of API/Ajax requests that are heavily used. With that, we've been able to eliminate more than a few Web servers that otherwise had to load the entire Apache/PHP stack just to serve a small 20-100 byte request.

With your response in mind - I don't see Node being mature enough (yet) to solely support an end-to-end large scale Web property. But, for serving up API content from Memcache/MySQL - it is blazing fast with minimal footprint. On our stack - we run Node right on our existing Apache Web servers (behind haproxy).