Not very, if you make sure you understand what you are doing first (but see karandpr's post below).
The procedure depends on the device. It's best to ask in your own device's All Things Root forum. There will be a sticky post there, All Things Root Guide, with home helpful links, and of course you will find information in the forum itself.
If you have any doubts at all, ask for advice before proceding. Better to avoid a mistake than to have to recover from one.