I've been using ThinkingRock at work on Windows and home on Mac (both v1.2.3) for about a month now. It's lovely. Thanks so much.
I had three criteria for a GTD support app, after going through several iterations of other things and ThinkingRock meets all of them wonderfully.
It's basically canonical with little or no extra stuff. This is because everything outside of the canon is someone else’s 20%. (You all know about the 80/20 rule, right?) Everyone has their own ways of doing things. But I find as long as I've got just the canonical stuff it’s easier to do my specific preferred 20%.
Allows tight-coupling of next actions with projects. This is neither here-nor-there canonically as far as I can tell. Some people don’t find it necessary. I do.
Knows about dates, allowing tickler-file-like behaviours and calendar-like behaviours. Dates and calendars are tricky things. Every one of my previous iterations didn’t do the date-related stuff that I wanted or didn't do it in a useful, usable way.
I find that ThinkingRock works for me by getting out of the way. Other "solutions" I've used have always left me feeling like I was fighting the app to get it to do everything I wanted. ThinkingRock works for me by disappearing -- I don't have to think about the tool, I think about what I am doing with the tool.