If you're a newcomer to the Mac or to Windows virtualisation, Parallels Desktop 9 is an easy sell. If you need to run Windows on Mac, this is a fast, usable, stable and configurable way to do it. As long as you don't want to play particularly recent games, Parallels Desktop 9 performs admirably when running almost any Windows applications you care to throw at it.
You can run apps full-screen, across multiple monitors (set-ups are now 'sticky' between locations), or with Windows and Mac on the same screen.
If you're upgrading from Parallels Desktop 8, the question is whether the new features are worth the price. There are plenty of nice ideas, but little wow factor. Support for new operating systems (OS X Mavericks and Windows 8.1) is the big draw, but Parallels Desktop 8 works fine with betas of both.
Desktop 9 offers performance boosts for shutdown or suspension of virtual machines. You also get printing to PDFs (which worked fine during testing, albeit really slowly), PowerNap support, cloud-service sharing between VMs and OS X (to avoid duplicate caching), Thunderbolt and Firewire drive assignment (as per previous USB drive functionality), and a "Windows 7 look" for Windows 8.
The last of those is an integrated installation of Stardock's Start8 and ModernMix. Use Boot Camp and the Windows 8 install will bug you to register, but in Parallels Desktop, the add-ons enable Modern UI apps to work in Coherence mode rather than forcing you to full-screen.
Parallels Desktop 9.0 is a reliable and convenient way to run Windows on Mac. It's not an essential upgrade: despite some useful touches, Desktop 8 users will see little improvement. But newcomers will find Parallels Desktop 9.0 robust, reliable and thoroughly up to the job.