I bought the add-in about two years ago, just after I discovered the GTD methodology. Since I use outlook, I thought it would be the fastest way to get up and running. I still think so.
I use the add-in every day. I think it is worth the money.
The add-in lets me focus on the work I do, not on the system. Since I did not devolop it myself, I don't spend hours and hours tweaking it (which I probably would do if it were my own, homemade system).
It has limitations, but I have found that practicing GTD within the limits of the add-in is an efficient practice. The add-in automates a few basic tasks like delegating a next action to someone else and creating the @waiting_for next action with a consistent format of delagatee and original date. The add-in also sets up task views to let one see actions filtered by project or by context. I use the 'by context' view frequently to see what I could work on next. I use the 'by project' view weekly as part of my weekly review.
It has tools to help administer a project list and an someday/maybe list.
I think it also gives one the ability to learn GTD without also having to learn to set-up and maintain a system.
There are keyboard shortcuts that let me capture an unimportant task or errand quickly and easily, freeing up my mind for the more important things. An example of an unimportant task would be 'buy tarp to cover wood pile.' The same shortcuts let me capture important tasks, too
I can sync to my PDA so I can review my next actions and checklists while away from my desktop PC.
On the negative side,
1) There is always room for improvement - new features, ways I wish the add-in functioned.
2) The web site forum is pretty lame. The developer of the add-in does not typically participate, so there are many unanswered posts.
3) Some users have had problems during installation.
4) You really do have to conform to how the add-in works. It won't bend to fit your will.
5) There is no version for a PDA. Nor is there any easy way to keep two computers running the add-in in sync.
6) Since the add-in takes care of many things 'behind the scenes' I had trouble trusting it in the beginning. I can see that if I had manually copied an email to the @action folder, I would trust that it was there. With the add-in, I don't participate in every step so I must trust that the steps were done properly. Of course, the automation is the main reason to use the add-in so I wouldn't want it any other way.
Ken