Emptying my email inbox was, by far, the biggest hurdle to implementing the GTD system for me. I had about 8200 messages in there! After about a week and a half, I had managed to "get in to empty" but not without having considered giving up several times
I average around 200-400 messages per day (about 70% of which is actual corespondence I have to deal with, the other 30% is spam and various lists, etc.) so email is a huge part of my workflow.