I use OmniFocus and not Things, but if you are serious about GTD and put your life in the hands of an app, $50 is absolutely nothing. OmniFocus could be $50 a m

I use OmniFocus and not Things, but if you are serious about GTD and put your life in the hands of an app, $50 is absolutely nothing. OmniFocus could be $50 a month and be worth it most of the time. (I do a lot of traveling, freelancing and I am rather scatterbrained to begin with.)