I've used Parallels and found it OK, although their upgrade policy is expensive... you can pretty much count on paying them 45-60$ at least once a year if you are religious about keeping your Mac upto date (almost any significant Mac OS upgrade requires a paid Parallels upgrade). Although Parallels updates include feature/speed additions/upgrades, they also include support for OSX upgrades, so you're forced to upgrade.
VMWare Fusion (the name of their windows emulator for Mac)is also "paid", seems slower than Parallels to me, but also seems more stable and less buggy. VMWare is also far more reasonable about upgrades. OSX compatibility upgrades are typically free... it's the feature/speed upgrades that typically cost, so if you're just looking to keep what you've got, you can go a fair while without paying for an upgrade.
Virtual Box is free, open-source, and apparently much more difficult to install. Much more feature-limited, and also does not support booting Windows off an existing Bootcamp install... something that Parallels and VMWare do support. This might be important to you if you dual-boot, as using Virtual Box will force you to have a bootcamp parition/Windows install, and also a virtual machine for Virtual Box to run on, and never the two shall meet (e.g. if you install Manager in Bootcamp Windows and Virtual Box, these are two completely different, unrelated installs). In certain scenarios, you might want this... perhaps you might want different versions of software running on the same machine? This is actually the reason I've never tried it. I only want a single windows install.
In summary:
Parallels: somewhat buggy, fast, feature-laden, well integrated with OSX, expensive to keep upgraded
VMWare Fusion: stable, reliable, less features than Parallels, reasonable upgrade policy
Virtual Box: free, feature-lite, works only in virtual machine mode, does not support running off an existing bootcamp install. Difficult (nerdy) to install and configure (for some of you this might be a 'feature').
my two bits.
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