I've been using Things for the last two weeks or so as my GTD application of choice. I was previously using Backpack for the same thing, but having been caught without a working internet connection a few times had me on the lookout for a local app.
Things is, or will be, almost exactly what I need. It's not so complex that I get mired down in the work of keeping myself organized, but it's flexible enough that I've figured out a way to compartmentalize everything as I like it.
I'm not convinced of the usefulness of tagging inside the application yet. I've been faithfully tagging my projects and todos, but I have yet to actually use any of those tags to find or organize things. Sorting things into Projects and Areas seems to be sufficient for my needs.
I'm looking forward to the final release, and plan to put down my $39 when they give me a chance.