I have been using Parallels and Fusion for several
years and am quite happy with the performance.
Lately, however, been sticking with Parallels. It's
performance with Windows 7 is much better than
with Fusion. Fusion, on the other hand, seems to
have better support for Linux systems.
Currently using Win7 Pro/Parallels on a 2011 Mini Server
with 8 gig RAM (you will definately want that) and the
Apple Thunderbolt Display. I also use the combination
on my 2011 13" i7 Macbook Pro, which also performs quite
well.
As to my use of Windows, I am an electrical engineer that
develops all manner of hardware/software/firmware which
gave tools only available on the Windows platform. Everything
from embedded tools from Cypress/Microchip to heavier duty
tools from Xilinx to apps like Matlab for data analysis and
simulation. I even use the LinkInstruments DSO-8502 USB
based oscilloscope (500 Ms/sec) with out any hick ups in
the display.
However, most of the work I do is CPU intesive, which Parallels
manages to leverage quite well. But, if you require heavy duty
graphics, the virtual environments leave a lot to be desired. They
unfortunately cannot leaverage the Mac's GPU directly and have a
sort of emulation layer. So, for graphics intensive games for example,
the virtual environments will severely underperform.