[Arild]

Having tried Things for a month, I totally agree with Toadling and MutantSquid. The tags and the way they are hierarchically ordered in Things are great - for reference. But not to get things done. GTD requires some self discipline, because it is tempting to jot down all associations that comes to mind around a task, but with the information overflow most of us are exposed to, that is simply more work than most tasks require.

To start with, I overused OF, wanting to manifest the entire process of thinking and work "inside" the program in stead of focusing on what it does best, which is keeping your personal workflow clean, across areas of responsibilities.
Today, I use Evernote before I know if I need to act upon information, and I use tags there for future reference. If a specific tag or combination of tags occurs a lot, that fact might prompt me to take action - in OF...
But there are lots of ways to get ahead; OS X' native notes, Bento, perhaps mind mapping software, or, of course, Omni's Omnioutliner for stuff where you need structured reference more than associative help. Most Mac software has a trial period, so you can try out what works best for you before investing too much time and money in it. But remember that if structured cross-reference is VERY important, a good relation database program is really the only thing that works 100% (provided you have the money or knowledge to customise it to your needs).

To sum up: My experience is that keeping your "actionable items" as simple as you can is the most important, and for that purpose, Omnifocus works better than Things. Things really lacks tools for structuring the workflow, particularly in projects. With Things, you find yourself trying to replace those missing tools by adding tags and then you keep asking yourself if you are ready to start action until it is too late anyway...