I can confirm the FX Chain/ProChannel issue, but everything else 6.4 related is working here perfectly so far. I'm even running Apollo, and it rocks with Sonar.
The only glitch I've found is that if I try to load more plug-ins than the DSP can handle, Sonar freezes and requires a restart. Trying the same thing with Pro Tools 10, Cubase 6.5, and Studio One Pro 2 gave error messages, but they kept working.
As to FX Chain thing, I wonder if the plug-ins living off in a separate hardware-land, instead of running native in the computer, makes a difference...maybe it's like when bouncing using external hardware, and you have to bounce in non-real time.
Danny - what's the nature of the problem? I may be able to help. I gotta say that going from 32-bit to 64-bit UA version 64 has been totally painless, and that includes installing, updating, and authorizing an internal Quad card, the card in the Apollo, and an external Satellite. I was pretty impressed it all worked so well, actually.
The lack of "qualification" doesn't mean they won't fix Sonar-related issues, but it also means that someone in tech support can't call up the database of test procedures they used for Logic, Cubase, and Pro Tools. It also means that particular person may not be familiar enough with Sonar to understand the nature of the problem.
So this forum is filled with cool people, let's see if we can help you