Package image is courtesy of Parallels. All other images are the author's.
Unlike some of its free competitors, such as VirtualBox, Parallels has a price tag (after a 14-day free trial). With that, however, comes an unmatched level of integration between OS X and the virtual computers that it hosts.
Parallels Desktop 11 comes in standard, Pro and Business editions. The first of those costs a one-time price of $79.99 (plus the price of future upgrades), while the other two are $99.99 per year. Probably the most important differences between them for most users are the amount of virtual RAM and virtual CPUs available for VMs (8GB vRAM / 4 vCPUs in the standard edition, 64GB vRAM / 16 vCPUs in the other editions) and the availability of Parallels' remote access service, Parallels Access (it is included in Pro but not in Business and the standard version offers a 3-month trial).
The majority of home users who just want to run Windows programs on a Mac and possibly dabble in Linux or Chrome VMs wont need to worry about technical differences, however, because the standard edition has enough features to handle home office and general productivity software such as Microsoft Office. Parallels Desktop 11 can also host an Android VM but this remains an experimental feature and lacks access to Google Play.
As with Parallels Desktop 10, it's a breeze to install VMs in v11 thanks to an inbuilt wizard that can locate or download installation files for Windows, OS X, Android or Linux and take you through the set-up process in a few steps. These steps involve choosing a location where the VM file will be stored and setting specifications such as hard drive size and amount of RAM. For general purpose uses, though, the default settings will usually be sufficient.
Parallels Desktop 11 has been designed to support new features in Windows 10, including Cortana, Microsoft's answer to Siri on Windows phones. Whenever you have a Windows 10 VM running in Parallels Desktop, you can access Cortana through voice commands and ask it to search the web, give updates and answer questions about the weather or your upcoming appointments. How much use this is depends on whether you buy into this kind of feature but if you love Siri, you can have a substitute while you wait for Apple to make its personal assistant available on a Mac.
Unfortunately Cortana couldn't resolve some of the small niggles I encountered when running a Windows 10 VM under El Capitan, Apple's own recent OS upgrade. For example, the Windows Start Menu has a solid colour background when it is stretched across the entire screen in Parallels Desktop's Coherence mode (explained here). Under Windows 8.1 the menu was overlaid on a blurred image of the Mac desktop, which was more appealing to the eye. It looks that way in Windows 10 on a standalone machine, too, so it's a shame this feature is not reproduced in Parallels Desktop 11.
Other issues include Microsoft's new Edge browser freezing when I tried to launch installation files from its Downloads menu, the Windows VM icon failing to disappear from the OS X dock even after the VM has been shut down and the Start Menu sometimes not appearing when the Parallels Desktop icon is clicked on. With Windows 10 and El Capitan having only been released recently, however, such bugs are to be expected and will hopefully be fixed over time.
Parallels Desktop 11 can be configured according to whether your primary use will be productivity or gaming. If all you plan to do is run office software or other software that doesn't make high demands on the VM's virtual processor and graphics card (and therefore the host Mac), the former should be adequate. The latter setting lets Parallels Desktop prioritize those resources that are most important for video games but even then you shouldn't expect to do hard core gaming in a virtual machine. The limitations are ultimately imposed by the Mac and building gaming PCs has never been Apple's strong suit.
There are lots of other settings you can tweak in Parallels Desktop, too, such configuring the network connection so VMs are recognised as separate computers on a LAN and setting up Parallels Desktop itself to respond to preset voice commands. Most of these will only be of interest to advanced users with special requirements but they are worth exploring through Parallels' helpful web-based user's guide and its online troubleshooting guides. The latter are a useful resource if something isn't working right but some could do with updating since they don't mention whether or not they apply to the most recent generations of the software.
Unlike VirtualBox, which requires you to set up shared folders between the VM and its host, Parallels Desktop 11 allows all folders and attached drives on the host to be shared by the VM OS by default. Opening a file in a host that was created on the client (or vice versa) is therefore as easy as if both were saved under one OS. Together with Coherence mode, this makes it possible to run Windows and Linux software on a Mac seamlessly. If this is the level of integration you need, Parallels Desktop 11 is well worth its price.