VirtualBox has improved a lot over the years, of course it's not perfect, but it's a decent, usable product. It's great for trying out new OS's, or apps that applies to the OS that you're currently using. If say, you're running Windows 7, and you wish to know how a particular software will act on it, having an extra Windows 7 install (you can run 7 for 30 days, for free) can help in a decision.
It's also good for those who doesn't want to let go of prior OS's, such as Windows Vista, XP & 2000. Or trying out Linux OS's before install, to see if it's right for you, w/o the work of a full install.
As far as security goes, one must have security software installed in a VBox install, just as any other virtual machine software (such as XP Mode). If the machine is going to connect to the internet, it needs to be secured. But as Joe pointed out above, if a problem (such as a virus) arises, 2 or 3 clicks deletes the entire VM.
As to whether it's the best VM solution, users experiences varies, and it's a matter of personal preference as which to use. VBox meets most of my needs in this regard. It's the 1st choice for the majority of Linux users to run VM's in.
Cat