It really depends on how complex your needs are. If you need a basic list to check things off and "brain dump" into to be sure nothing is forgotten, I think I'd just stick with the to-dos in iCal or Mail.app. They are basic, but fully integrated, no separate program to launch, and easy.
If your needs are more complex, though, and you're dealing with projects, scheduled future events, recurring events and in general just need more robust task management, I do think Things is great.
I balked at the price to begin with, as it seemed overly simplistic to me at first. I downloaded the trial, tried it, watched some screencasts about it to get an idea of how people use it and I quickly started to really appreciate it. I also tried OmniFocus, the Hit List, Midnight Inbox and I'm sure a couple others. OF and MI were just too GTD-structured for my workflow. The Hit List was pretty good, but with no iPhone app and still being in beta... just not at my comfort level for an app that is "mission critical" for me.