As I commented on Michael's blog, I'm partly to blame for The David Allen Company's success with Lotus Notes. You see, when consulting for my clients, I've always focused on the end user need and allowed that to drive the architecture, software selection and implementation. I simply gave my clients whatever they needed to get their job done and I tried not to let myself of the technology get in the way. As a result, for those of my clients that use IBM Lotus Notes, Notes became a big win for them. While this is not be a radical approach, I'm often surprised to see how some implementations are done; as a result, people often develop differing opinions about Notes.
Anyway, Michael Sampson's post is a good one and worth a read:
David Allen Comments on the Power of Lotus Notes for Ad Hoc Databases