VirtualBox has one advantage now, and that is that it is licensed at no charge. On Linux, this isn't a big deal (as KVM and Xen are decent alternatives), but a hypervisor on Windows or OS X, this can be important.
However, if one can choose a non-free solution, the competition has lapped VirtualBox several times. VMWare is extremely strong, both with Workstation on Windows or Linux , as well as Fusion on Mac. For a dedicated box with a tier 1 hypervisor, both Hyper-V (can be downloaded separately from Windows) and ESXi are quite useful (although there are limitations without the commercial management tools.)
I've tried various VM products, and the main reason that I chose to just go with VMWare is the universal-ness, and because it is at least a generation past the competition with dealing with RAM overcommits, snapshots, clustering , and other features. Plus, if a company sells an appliance, it almost always will be distributed as an
Hyper-V isn't bad, as the latest iteration auto-activates Windows VMs sitting on it (no need to worry about a KMS server accessible by all VMs... just the operating system instances running on bare metal). However, usually it is implemented with the full Windows Server OS underneath, making an attack surface, as well as a point of downtime. However, for a Windows shop, the price is right, and it does a good job. VMware is great... but you do pay a king's ransom for the features it brings with it.
: If one needs a home machine to run VMWare stuff on, one might be better off running VMWare Workstation ontop of Linux because ESXi cannot use USB hard drives as backing stores, while VMWare Workstation really doesn't care since it is a type 2 hypervisor and lets the OS handle the disk stuff. Of course, don't expect vMotion or other stuff... but if one wants a dedicated box just for virtual machines, this is a usable alternative.
: Clustering and fault tolerance is brain-dead easy, either using VMFS on a logical drive from a SAN or a NFS backing store.
It is the only free desktop oriented virtual machine to have versioning of clients so you can roll back vms. You have pay for that in its competitors.