Edit: some of you have pointed out that I can resize a non-zoomed track and it resizes all of the the non-zoomed tracks. Works perfectly, and is a very nice feature.
I love the idea, but the implementation in X2 is not quite usable for me. When Auto Track Zoom is on I can't completely minimize all of the non-zoomed tracks, which results in waste of real-estate. Bummer, because Auto Track Zoom would be very nice if it didn't have this problem.