I always used VMware, I should give a shot to Virtualbox, but coming from Oracle it's what makes me lose the trust. I know it hasn't always been from Oracle, but after seeing the Java stuff they have done (like patching and then finding out the patch wasn't really there), it's hard to put confidence on it.
If you have a chance, try ESXi (unlike ESX), it's free last time I checked, it runs on the bare-metal (no underlying OS required), less than 120 mb footprint, it's not based on any OS, hypervisor... blah blah. You can use that if you have a dedicated box and then want to run all your vm's there. It's a nice setup if you want to look into researching under a safe environment (malware reversing, etc.).
We should actively fuzz Virtualbox, not to put Oracle to the public shame again, but to make sure they do their homework. That's just my personal opinion.
