I also find the rear rack fairly useless- The heel strike issue is ever present.
I find the rack is quite useful - not for panniers but you can fit a rack pack or use the bungie to secure odd items. I mostly use it to carry the 'el bolso' bike bag, which I have to use to persuade my building manager to let me bring the bike into my office (there's a blanket ban on bikes in the building but my argument is that, once bagged, it's just a piece of luggage). For carrying other stuff, including my laptop and other work stuff, I use the excellent Carradice SQR seatpost bag ('Slim' version). This clips on and off in a second or two.