What is the performance?
The performance is identical to a standard installation, except for hard-disk access which is slightly slower than an installation to a dedicated partition. If your hard disk is very fragmented the performance will degenerate.
Hibernation is not supported under Wubi, moreover Wubi filesystem is more vulnerable to hard-reboots (turning off the power) and power outages than a normal filesystem, so try to avoid unplugging the power.
An Ubuntu installation to a dedicated partition provides a filesystem that is more robust and can better tolerate such events.