I just signed up with Astrid this week, and as a result of doing that was inspired to reconsider using GTD principles after some time away from it. I previously was using RTM, before that Toodledo. So far I think I like Astrid better, with one caveat: I'm an iPhone user and, though there is a fairly nice iPhone/iPad app, I didn't know before I signed up that the primary mobile platform the developers are focused on at this time is Android. (I signed up at the recommendation of and Android user.) That means that new features appear to be available to Android users first, with some lag for iOS users.
I've spent a good deal of time this morning setting up folders and lists that accommodate my current understanding of GTD, and assigning tasks accordingly. Time will tell if I can make it work efficiently.
Having gone through this process makes me wonder if I could do the same with RTM--I never tried. I might go back and take another look at it from that perspective.