As You can see from the former reviews opinions really differ on this app. To me in contrast to the rest this is the best GTD app for Your mac/iphone, especially if You also have the Mac application.
In comparison to OmniFocus, which I also own, Things is not giving you the restrictions other GTD software requires to work- although the tagging features and lists make this app an intuitive pro planner for Your GTD system You absolutely do not need to use it that way. So happily implement Your own workflow and enjoy the freedom Things is offering You!
In contrary to the other opinions I think the programmer are doing a GREAT job in updating the app with useful features while keeping it fast, clean and not as overloaded as OmniFocus. Syncing is a breeze and considerably faster than OF, while data entry is as structured and detailed as You want it to be- point taken, if You are using this app and not already have a good GTD discipline Things for the bad of it all will display exactly this type of structure- but exactly this mirroring actually helped me visualize mistakes in my own planning and helped me overcome it thereby!
So, for me at least, this app is sitting in the menu row of the iphone and is indispensible!