The problem with Things and OmniFocus (for me anyways) is that there's no web based client or Windows client.
I bought OmniFocus a while back for my Mac/iPad/iPhone but I haven't really been able to use it as much as I'd like since I use Windows at work. Even a simple web based view would cause me to get a lot more use out of OmniFocus. If Things had web based access, I would buy it in a heartbeat.