I would recommend at least having a look at 5.5 SOHO Organiser. They have merged the contacts and calendar parts and have started on building their own views. So there is now a day view and week, month, year etc views are coming. Speedwise it is OK but I have a MacBook with 2 gig RAM so I would rather hope it was fast.
The reason for linking in with iCal and Address Book is they decided to give up on their own syncing and use Apple's built in sync services.
At its present stage of development I don't find it as good as Personal Organiser. Part of the blame for this of course rests with Apple for deciding to give away free a pretty good calendar and a pretty good address book app. This means third party developers lose a lot of potential market. I think SOHO Organiser will be worth buying when they get all the other calendar views in place.
I also switched to MarkSpace's Missing Sync for PalmOS at the same time. This has many advantages in particular network syncing and background syncing. The Palm HotSync stalls if you put it in the background and Palm don't support network sync on the mac.
For those worried about converting I converted about 4,500 events from Personal Organiser and about 150 contacts without any apparent problems.
I use the Tasks on my Palm for GTD. The Tasks view on SOHO Organiser isn't very exciting but neither was it on Personal Organiser.