What can be said has mostly been said, but I'll chime in. It's not quite as nice to configure as Parallels, but once it's running it's fantastic. Two-way clipboard support just works, and the granular control over the emulated hardware (how many cores, how much RAM, what size drive, etc.) makes it sweet for testing system parameters. I triple boot on my MacBook, but may toss that for using virtualized PCs.
It works well with most Linuxes I throw at it (have had some issues with Linux Mint and video drivers). It works well with Windows. Parallels seems to virtualize Mac OS more smoothly. For free (I, too, was tired of the "subscription model" annual upgrade pleas/threats), I really have to try to find a problem with it.
I sometimes wonder if VMWare Fusion would be worth it, for the raw device support, etc., but I understand that it can be configured in VirtualBox, with patience and Terminal-fu.
In short, it does so much of what I need it to do that the commercial competitors don't have an appeal for me. I may, eventually, see if qemu is faster, but that's even more arcane, so it's not likely in the short term.