I've been a GTD user for a long time. Personally I think Toodledo is a great site, but misses some of the core GTD stuff.
I use a number of GTD iPhone Apps. By far Todo is the best for me as it's intuitive, a pleasure to use and not to many sub windows. Todo also supports sub tasks irrespective of whether you have a toodledo pro account. By-the way the sub task of the pro account is a real let down. To be really effective you need to be able to create sub tasks of sub tasks. This is especially important for big projects Todo does not have this feature either).
I also use a Mac and Omnifocus is well worth a look. However, the Omnifocus iPhone app is fairly useless without the desktop app. I find ToDo is the one I'm constantly coming back to and rapidly finding helpful.
One other caveat with Todo is the notebook app that integrates really well with Todo. there's even an app called Short Hand that allows you to create shortcut so for example if you type 'adr' it will replace it with a multi-line address. You can then export this straight to notebook or Todo. It simply means that if you're out and about you can actually enter in lots of text using your iPhone. Beware however that Short Hand has a few bugs that need fixing before I'd buy it.