I have been using Datebk5 for about 3 years now, and it's by far my most used Palm application of all time.
I was looking for a program to keep all my projects in as I found Datebk5 not quite suitable for that purpose (although I had seen many ways to adjust it for that use in many posts in the Yahoo GTD group -- somehow I never liked any one of them). I then started using ShadowPlan -- which was and IS an excellent application, but I never seemed to be able to fine-tune it to my liking either.
About 6 months ago I decided to give Bonsai a test-drive and I haven't parted with it since -- I really like how it links items to the Datebook -- I find it more intuitive than ShadowPlan. Thus, I now do all my GTD in Bonsai/Datebk5.