Sidekick is great for people who want to take their data on the road using a small handheld computer device, like the REX or PalmPilot. Day-Timer Organizer makes great sense for those already addicted to a paper-based Day-Timer system. This is not to say this program doesn't stand on its own, but traditional Day-Timer users will certainly find it useful.
Finally, if I were embarking on my first PIM, I would probably opt for Info Accelerator. This program makes it easy to embrace this new way of working because it's readily available within all your applications and is thoroughly un-intimidating.