Not that impressed. I had a MainGear rig that was "worth" (cost my employer) $6,500. Could it edit, yeah, could it render, yeah. What a waste of money though. It was slow at editing. I'm not lying either, it just wasn't configured correctly, and I was new to editing so my recommendations for buying a rig was thrown to the way side and they trusted the "experts". Well, so much for that. Just remember, MacPro's use Xeons... server proc's not just normal, and Bulldozer in it's current form, if I'm not mistaken is strictly desktop.
Edit: PPro was pretty frustrating too. Crashed a few times a week on me. Anywho, don't believe any of that PPro accelerated encoding crap. You basically have to jump through hoops, on a night there is a full blue moon, on the 22nd of the month, at a location that is in alignment with the planets, and there is an anomaly in the stars. I think you get my point.