I started using Things a few weeks ago and really like it.
I am currently reading David Allen's "Getting Things Done" and find that Things helps me get into the GTD mode without being overly restrictive.
I tried iGTD (both version 1 and 2), but found it to be more cumbersome to use.
I find that the biggest problem with any religion is that implementation tools become way too complicated for someone just getting started. The developers of Things have realized this and created an application that is simple for beginners and is also sophisticated.

By the way, I think the price is very reasonable.