Advantages:
- I get to tinker with multiple OSes (something I enjoy)
- if I'm about to go to a dubious website, I can clone a virtual
machine, use it for the dubious visit, then destroy it

Disadvantages that I've thought of so far:
- memory usage
- speed reduction
- hard drive usage for disk images
- complexity
- mounting USB sticks on guests is extremely problematic
- playing sound/video from guests through the host is imperfect
- hard to determine where an application is running from