dunno leh, i tried and it worked for certain situations. of course there will be a point whereby the room is just too big/ceiling too high and no amount of exposure compensation will help.
perhaps it worked in some of the situations that i was in because the flash was under-exposing. can't tell you for sure the reasons cos too many variables involved (e.g. layout of the room, reflectivity of the surfaces, colour of the surfaces, objects in the room, angle of bounce, distance covered, ISO used etc.)
in summary, my point is that if you are not flashing directly (e.g. bouncing), get the most powerful flash available. because the most powerful flash available in canon's lineup is still not powerful enough in my opinion.
a small aside: i tried using flash on a car that was about 50m down the street (think i used ISO 400). it came out fine! tried flash on this big bungalow opposite the road (about 20m away). lit up the whole bungalow! so, the 580EX is very powerful...but still not powerful enough for bounce photography in large rooms (e.g. banquet rooms with high ceilings) or mid-sized rooms with low reflectivity (e.g. matte finish wooden panelling).
Looks like mistaken technique more than equipment limitations.
It's like having a car that can only drive at a max of 150km/h. You can top up fuel, change the lubricant, wipe the windows, and maybe change your upholstery, and it'll still max out at 150km/h.
