The crucial thing for any given GTD apps is it must be anywhere but invisible. "Be anywhere" means you can access it anytime you have something popup in your head. "Invisible" means its operation (to add/remind task) must be less obstruction for you as possible. I blogged about this in Notification Console
RTM follows this approach by trying to be anywhere and it does quite well. You can access RTM via web, mobile, email, IM, twitter and so on. But RTM fails to be intuitive (read 'un-obstructive') to use. In other words, RTM has good concept in mind but its implementation sucks.