Different experience here
My experience is quite different. I use iCal and the Addressbook and have for years. I've never experienced any data loss from the addressbook at all. Ical's main issue I have is that I can't easily archive items in the distant past. I can delete all but repeat items but I can't do a clean archive of everything to reduce data size.
However, I do have some procedures to prevent any data loss.
First off I always assume my Mac is the trusted source for all data. So while I might add items in both calendar and addressbook to my handheld I sync as soon as I can and verify the data got transferred correctly. I also run Time Machine and store periodic backups on other drives located in separate buildings.
When I contact a customer I update my addressbook notes field with info about that contact. I currently have about 700 addresses in my addressbook sorted into 5 real groups and I also have a couple of smartgroups for special purposes. Any action items I get from my contacts I enter into my Omnifocus system.
For iCal I keep a detailed log at least to the half hour and some times of the year to the 10 minute of what I did every day. I often search this data to locate when things got done. I keep about 10 years of data on my handheld but try to archive or delete data older than 10 years to save space.
Where I have made the biggest change from most folks is my sync protocol. After trying Mobile Me I have determined that it is the source of nearly all the problems I have ever had with sync. Dropbox or other cloud sync also is problematic for the same reasons.
My handheld is an iPod Touch and I have it set to only sync addressbook and contacts via wire from iTunes. Keeping the sync to tethered by wire only has prevented any data loss. I have also noticed that when I boot up my machine in the morning if I try to start too many sync services too quickly I will get errors. So I start up Safari, then Mail, then iCal. Go start my coffee water. Come back and start up Omnifocus which I have syncing via bonjour over my internal wifi network. I have also noted that there appears to be a memory leak in the mac sync SW. If I leave my machine on all the time over several days sync gets more and more unreliable. The simple and easy solution is to shut down the computer at night and re-boot in the morning.
I have never tried to do anything with a Blackberry. Friends I know who have those devices do not attempt to integrate with the mac but instead get an iPod touch for their handheld and keep the blackberry as a phone only. My phone is a Palm Treo, but will be replaced by a simple dumb phone when it quits as I never use the smartphone aspect anymore.
Maybe there are some suggestions you can use from this.
Oogie McGuire - Mac, iPhone & Omnifocus
OogieM on Twitter
Paonia, CO USA