I don't have numbers but in general I'd call it no less usable than X forwarding over ssh. For a long time I used to connect to a Japanese Plan 9 server using d

I don't have numbers but in general I'd call it no less usable than X forwarding over ssh. For a long time I used to connect to a Japanese Plan 9 server using drawterm (a Windows/Linux application which essentially emulates the /dev/draw infrastructure) and it was pretty decent considering the latency--I used it to read email, follow IRC, and write code. Over a LAN, it's so fast as to be indistinguishable from something running on your local machine.