No word on midi, vsti, or X64 so far. The rumour mill says that all of that should be coming along soon. From my experiences today it just is a breeze tracking in X3 and exporting the OMFs in to Mixbus. I have always used Sonar from beginning to end on all my projects but hearing the mix and mastering in Mixbus seems to have more of a blended vintage sound that has been lacking in Sonar. I am not sure that I would go to the trouble of using Mixbus on a regular basis but it sure is nice to have the option of using it to knock of some of the digital brittleness I have in tracks that get sent to me. There are a ton of videos out there about Mixbus. Check 'em out.
BTW, I was loading projects in X3e 64 bit and exporting them out as OMFs, opening them, and mixing and mastering in Mixbus 2.5 which is 32 bit. I never had a problem over 4 hours of continuously throwing stuff in to Mixbus.
If this thing goes in to X64 with midi and vstis, I believe it will be a very big contender if they keep the price down. I also should calm the rhetoric. You know how it is when you get a new piece of gear. My hearing may change after a couple of days and the shine has wore off. LOL