I use mine on my 550D and am pretty happy with it. The 360AFD does come with a built-in optical slave mode, however in practice I can't see how to use it with the 550D's built-in flash being used as the trigger. As I understand it, the built-in flash on the 550D, sends out a preflash to 'test' the exposure. The pre-flash set off the 360AFD and it can't recharge in time for the main flash from the built-in flash, which is timed to go off when the shutter is open. I guess the only options are a dedicated hotshoe cable or preferably a set of flash triggers, which I believe only let you use the flash manually (unless you pay for the far more expensive ones). I've seen CT-04 and PT-04 trggers on Amazon (is there a difference?) without wishing to hi-jack this thread, can anybody recommend either?
I got my 360AFD from Jessops last Xmas for £59.