Had a few problems with Parallels on the Mac while meanwhile finding Fusion to be fast, functional and stable. So Fusion has been my choice. I've tried Parallels a few times just to see if I was missing something and each time went back to Fusion.
But I must confess that another reason I've preferred Fusion is that I used Parallels on Windows years ago and found the company's support to be absolutely terrible. Perhaps they treat their Mac customers better. It's not like VMWare's support wins any prizes, but Parallels treated their Windows customers like absolute crap, either ignoring them or making promises they would subsequently break over and over and over.
I also run a specialized Linux virtual machine in VirtualBox on my OS X server. It's entirely satisfactory, but most of my Fusion VMs need very high performance USB connectivity for one reason or another, and Fusion's USB functionality has been good while VirtualBox's is said to be somewhat less so. I haven't verified that for myself, though. Anyway, VirtualBox is free and might be worth a look too:
https://www.virtualbox.org
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Thanks for that, I was going to upgrade to mavrics this PM, not anymore.
Actually if you use VMs on a laptop there is very good reason to go Mavericks.
First, Fusion 6.0.1 runs really well on Mavericks. Mavericks' new memory-management technology keeps its footprint compact. And Fusion supports Mavericks' App Nap functionality, which will significantly improve battery life when running a VM. Since VMs (especially running Windows) tend to be terrible power hogs, this is a significant benefit.
Of course, as another poster noted, there are issues with Win 7 in Parallels on Mavericks, but that's a Parallels issue and can be addressed with an upgrade.