For me, Up is worth a significant premium because the software actually works and is relatively easy to use
I agree 100% - (almost) anyone can build a cheap 3 axis machine and tell you how to install and configure all the opensource stuff you need to make it work (Arduino drivers, Python, Skeinforge, Pronterface etc...) but the beauty of the UP! is that it's all done for you - unpack the machine, install the s/w, align the table, set the height and start printing.
I'm sure that's what most people want - they don't want to worry about the hundreds of settings in Skeinforge.
From experience, you really don't know where to start if your prints aren't coming out that well - is it the software, your settings, firmware or mechanics at fault ??
The only downside I see with the UP! is it's inflexible, eg, you can't just buy any old ABS filament (or PLA) and expect it to work