* Very easy to use software in general, with good user interface
* Good performance – as long as there's not too much going on on my host machine (MacBook Pro 2013), the Windows virtual machine runs almost just like it were installed on the actual machine
* Options for configuration (keyboard, mouse, network, external devices, etc.) are extensive – how they interact with host and guest systems can be controlled to suit pretty much any environments
* Excellent integration with Mac – things like shared folders & drives, shared applications, and "coherence" mode can make the experience with virtual machines very fluid
* Regular updates to fix bugs, enhance performance, and other improvements
* Support for wide range of guest OS's – good performance on all
* Good online support for all sorts of issues – email, phone, and remote desktop
* Complex key bindings (translations) sometimes to not get translated well – this has been my experience with Windows guest OS's at least
* It sometimes takes a fair amount of effort and several days to get support issues escalated to the appropriate level/team
Parallels Desktop definitely has advantages in integration, performance, and user support over free virtual machines. If you need virtual machine software for Mac, and are happy to pay, then Parallels is probably the best choice.
I use Parallels to run Windows 10 (and previously Windows 7) for various applications and for cross-testing software. Parallels has made this is a very straightforward task.