It's pretty universal and some of the features not implemented by files in traditional Unix (e.g. network connections, remote devices) work well in Plan 9. Though I think "everything is a file" is a misnomer in the context of Plan 9 - rather, everything is a name in a system of namespaces.
Interesting related paper which also gives some cases for which names don't work too well (one example is shared memory): http://doc.cat-v.org/plan_9/4th_edition/papers/names
