... it completely blew me away.
I left iGTD for good for some time now, I'm not religious enough about GTD to follow Thinking Rock, and I disliked OmniFocus' UI and workflow.
I stayed with TaskPaper for quite some time and was pretty happy with it - until I gave Things another try a couple of weeks ago.

Very flexible, great UI means I actually use it all day (and sometimes night ;) ) long, iPhone supoort coming - what more could you ask for.
By now one of the best and most useful apps I've ever used.