InfoWorld published a nice review of QEMU 1.3.0 rating it 8.5/10 and providing this bottom line:
Technically a simulator, QEMU nonetheless has virtualization capabilities that allow it to run Linux on Windows (or vice versa). It can also run code from one CPU on a system with a different physical CPU, so an x86 Windows system can execute a virtualized Sparc-based Solaris image.
QEMU is slower than commercial virtualizers, but it’s lightweight nature and platform support compensate for performance.
Read the whole review at source.