Ranked in these QuestionsQuestion Ranking
Pro Can run Windows apps as native OS X apps
Parallels has a "Coherence View" mode that allows running Windows applications as if they were native OS X apps. In this mode Windows is virtualized in background and Windows features like Jump Lists, Taskbar, Start Menu, etc can be embedded directly within OS X.
Pro Best-in-class gaming performance
Compared to other VMs, Parallels has the best gaming performance. Still, a good framerate can only be achieved with games that have very low resource requirements. A much better option for resource intensive games is dual booting Windows (via Boot Camp).
Pro Easy to use
Parallels has an easy to learn and use UI. All important features are wizard based. It requires only a few clicks for the wizard to download and set up the latest Chrome OS, Windows or Linux build.
Con Requires buying a new version of the software with each new OS X release
Chances are that the same version of Parallels won't work with different OS X releases, requiring a separate purchase of Parallels for each OS X version.
Con Very expensive
Parallels upgrades are very expensive. Even if I bough several versions of Parallels I will switch back to free VirtualBox as they even "invented" the Pro version - mainly doubled the price you have to pay each year to have the software updated.
Con Can't import OVA files
OVA files are useful because they include all of its supporting files. Unfortunately, Parallels doesn't support these useful files.