I use the Canon 580EX and the older 550EX mounted to the hot shoes on my Canon 1DsMkII and 20D bodies for fill flash outside most of the time. I set the flash power to -1 to -1.5 stops so that the resulting images are more natural looking. I have the flash set to high speed sync so that I can use any shutter speed.
The TTL system works very well and you can always simply turn off the flash if you don't need it, but it's there when you do. The 20D has a pop-up flash built in, however, the 20-70mm L lens with lens hood causes the pop-up flash to cast a shadow from the lens because the flash is not high enough on certain close up shots. Using the shoe-mounted flash eliminates this problem.
Kindly yours,
Bud James
North Wales, PA