I'm never going to ditch my Mac in favor of a Windows PC - I prefer the Mac user experience to the Windows user experience, that's all there is to it. My 14 year old has been pretty happy he's been dividing his time between various Steam games he couldn't play on the Mac and other fare a particular favorite is Smite, a multiplayer online battle arena strategy game from Hi-Rez Studios. For many games, that's frame rates of 20 to 30 frames per second for others, it's even higher, with higher levels of detail possible. If you keep your expectations realistic, you can usually tweak settings and resolutions enough to get a really playable experience.
Many don't, and the relatively modest integrated graphics in the mini are perfectly sufficient to play them. Obviously some games employ enormously sophisticated 3D engines that want more horsepower than the Mac mini can muster. A lot of it depends on the individual performance requirements of the game. It's optimized for games, and unlike the other two or Boot Camp, doesn't require you to have a Windows license. CodeWeavers' CrossOver is worth a special look from gamers.
Both continue to optimize their releases for the broadest and fastest driver support they can.
VMware Fusion and Parallels Desktop get the lion's share of attention, partly because they broad appeal for Mac users who also want to run business software and specialty apps that won't run on OS X natively.
If you're looking for a Windows game experience but are reluctant to have to reboot your Mac each time to do it, you can install software that bridges the Windows gap.
There are a lot of good games on the Mac, games that will run well or decently on the Mac mini. Running Mac games natively and exclusively is always an option. Once Drive Utility patched things up I was able to get started. This Mac's been well used since it was new, and its drive partition didn't pass muster with Boot Camp Assistant. It took a few afternoons to get everything squared away. Windows graphics tend to run faster than OS X graphics do on the same hardware, thanks to more extensive Windows driver optimization and underlying differences in the way the OSes are structured. Whatever Mac mini you use will probably run Windows games faster than it is does Mac games. Because while it spends some of its time running Windows, it's also a great general purpose Mac running the latest version of Mavericks. Its graphics speed can't compare to a dedicated gaming rig, but it doesn't have to. It may surprise you, but the Mac mini is an adept little Windows gaming system.