I only downloaded Things earlier today, but already I'm impressed. It was easy to set up my projects and tasks (as in, almost no time at all), and though it's still in alpha it's completely useable, if currently lacking some features (and most of these are promised soon).

It's certainly the most natural GTD software implementation I've come across. Like the previous poster, I tried OmniFocus and iGTD, finding the latter a disaster to use and the former too structured. It felt as though I had to jump through its hoops before I could enter my tasks (maybe that was the intention).

Things, on the other hand, has that clean UI feel and I found it didn't take long to work out what went where. The distinction between Areas and Projects is a little blurred, though, and this might be problematic. The use of tags instead of context categories that can't be ignored is a touch that makes it more flexible, and less imposing. For the GTDer, the Someday and Postpone categories are a nice feature (lacking with such clarity in other apps, I might add).

In short, Things is the first GTD app I've come across that actually makes me want to do my tasks (is that a little geeky?). It's by no means perfect, but I'm looking forward to future improvements.