There are existing NX implementations that work really well, though, e.g. OpenNX (client) + FreeNX (server). While xspice has potential, for now (and forward, should xspice not "get there") NX is a viable option.
One problem is that NX 3 doesn't support newer X extensions like COMPOSITE and DAMAGE, though. NX 4 does, but is no longer open. So the future viability is kind of starting to break down already, if you ask me.