I've had no troubles with it.
See here and
here. Taken with SW ED80, WO FFIV, and 40D. Not the best photos, but nice flat field to my eyes.
Logan (logieberra) tried it with a Pentax (I think it was) camera and had an issue with it clashing with the camera body, and also had some issues with the flattener itself that appeared to be one of the lenses inside it installed around the wrong way.
I'd think the 550D should be ok. Only think I'd check is the pointy flash lid thingy that sticks out over the lens - make sure that clears the FFIV. I have my wife's 350D at home that I could check to make sure it clears. Will try to remember this weekend.