X3 is a step in the right direction, in terms of stability and performance. That said, I've still run into intermittent (thus, non-reportable) but recurring bugs.
* ProChannel modules at random occasions mute the whole track. I've seen this happen with Console Emulator and Tape Saturator in different projects. Suddenly, no signal would pass through the effect. Turning it off would make the track audible again. Turning it back on would mute it again. Nothing in the effects' settings would cause the track to be muted. Restarting Sonar clears up the issue completely, until it occurs again.
* The ancient issue of a single MIDI note suddenly and randomly being sent to every MIDI track resulting in a loud cacophonous explosion of sound.
* Getting constant dropouts having opened an old project, even though neither the CPU meter, nor the ram meter, nor the disk meter, are anywhere near the top 50% of their respective scales. Increasing the ASIO driver's buffer does not help. No such issues with other or newer projects. Since the old project has bounced Hypersonic tracks, I'm tempted to suspect that audio contains NaNs or some other heinous things of that nature. I presently don't have a way to be sure.
Don't get me wrong, I'm still blown away by the awesomeness of X3. I just can't wait to be even more blown away by X3C. :)