Things is great, but the plot twist is that it actually costs $50 for the Mac, $10 for the iPhone, and $10 for the iPad -- all separate purchases.
The company also has a habit of charging $TEXAS for upgrades for new major releases. That is totally within their right and they deserve all the cash they can get -- but that reality may affect your buying decisions.
I did the Things thing for a while but determined the money leaving my wallet wasn't worth it for me. That said, it's among the best, cleanest, highest quality Mac software I've used.
0. http://culturedcode.com/things/
1. https://gettingthingsdone.com/what-is-gtd/