For MIFit downloads stats are 63% windows, 27% Linux (no fully functional Mac port yet).
I have been doing crystallography on moderately high-end Windows laptops for a few years now - mostly MIFit with CCP4 but SHELX, MOSFLM, d*TREK etc are also fine. Performance is NOT a practical issue as far as I can see - image data processing typically 5-10 minutes, refinements often 1-2 minutes.The upsides are that you don't need to think in terms of getting a 'special computer for crystallography' and have all office work, chemistry applications and software etc on the exact same disks. Everything generally just works from the installers so I would say you are more likely to be able to just get on with your structure-related science on this OS that Linux. One deciding factor is what software you need outside crystallography and on what platforms it is supported. I see a little bit of conflicting issues getting into the thread - optimal OS for a server or for development is a different question.
