The article gilead links to sounds like the reviewer was running QEMU without either kqemu or kvm - it was running fully emulated, and thus slower (and without sound enabled).
From my playing:
VirtualBox is slick, easy to use, fast, and adds some nice GUI indicators around the borders. It can easily make use of Intel's VT or AMD's SVM virtual machine additions, to speed things up. Unfortunately, the GUI bling means that a 1024x768 virtual screen won't fit on my laptop monitor (which is 1280x800) so I'd need to run it on my LCD second monitor (which I don't want to). It won't run everything, but it runs some things that won't run in QEMU with acceleration.
QEMU is slow - until you sort out acceleration, either kqemu or KVM. The base QEMU is run from a shell, and takes options to tell it how to behave. With kqemu installed, it uses acceleration on user mode code by default, and on kernel mode code if you ask it to. KVM is a modified QEMU that can make use of VT / SVM, like VirtualBox does. Not everything runs in QEMU with acceleration - I can't get Mandriva 2008 Spring installed in QEMU, whereas VirtualBox got it running easily. I prefer to use QEMU, however, because of the simplicity - it doesn't add scads of GUI bling to the virtual screen, and I'm not afraid of using the monitor (console) to control the VM.
There's a utility called Virtual Machine Manager that gives you simplistic GUI control over QEMU and KVM, as well as Xen. From the little I've used it, it is just too simplistic - although it does provide simple management of bridged networking if you require that.
Xen is another one you might want to look at, although I can't comment on it because I've never used it.