I use the Time/Design forms: activites, billing & accounting, project planners. I also use their databank which is great.
Because I use both my powerbook and treo to keep track of my schedule and contacts I don't need to use their calendar system.
Initially I purchased their outlook companion which is a pretty good system on it own (not to mention cheaper) however, I didn't like the time/design binders. I purchased the Filofax A5 planner at TJ Maxx for $15 and it is perfect for the time/design system (it has the same punch holes as the time/design forms).
To make the most of the system I did purchase the time/design 6 hole punch. Now I can put an 8.5 x11 inch paper into my system for reference if I need to.
Overall, it is a good system but there are a lot of other good systems as well. What I liked about this one was that it has a lot of room for customization.