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.
Definitely, I use NX myself daily at the moment. The Windows client sucks (slow, no free resize as in the Linux client, lots of clipboard problems), but Linux-to-Linux is quite awesome.
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.
Windows NX client treats me well. The lack of free resize hasn't bothered me yet as my workstation is really just a thin client with Outlook, so I just maximize the NX session.
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.