We used them so the wife and I could both (2) box (we own 4).
No more EQ for us, so 2 of them have to go.
Boxing with an extra mac - instead of 2 instances is much faster in most cases. Just use a KVM to control both at once. We used 2 monitors, one keyboard and one mouse. You could also use just 1 monitor, but we preferrred always seeing both and switching keyboard/mouse control with a hotkey. Two full machines beats running two instances and its pretty cheap.
See this craigslist ad for details. Of course, we'd have to deal with shipping.http://rochester.craigslist.org/sys/2821931136.html
Make me an offer!