This is the critical point beyond which I cannot look. If you have a car, it absolutely needs to do two things: Start and stop. Everything else can be compromised at least to some degree. Starting and stopping cannot.
The equivalent of a car starting and stopping is for a smartphone that the address book synchronization must reflect 100% fidelity. All changes and additions must appear on the other side, hopefully within a few short minutes. Not hours or days, let alone forever.
If a smartphone cannot pass this test, it's the end of the argument. Full stop. In my month with the Q10, it failed this test -- again, based on Gmail only, and a large address book.
After you reboot the device, or just shut it down -- say, overnight -- it takes a long time for the address book to return to life once you fire up the Q10. Initially it tells you that you don't have an address book. Imagine my panic the first time I saw this! But then, just wait perhaps a few minutes, and it has started rebuilding. When has it finished rebuilding? Again, with no number to track, it's impossible to say.
This leads me to the other problem with the Q10, and that's the less quantifiable issue of general slowness in accomplishing things.
BlackBerry's chief hallmark had always been how quickly you "got stuff done." For example, answer an email, edit an email, add someone to the address book, look up someone in the address book, and similar basic needs that you perform constantly.
A key problem of the Q10 is that almost all of those things now take more time to accomplish. Just try to copy/paste a piece of text, say in an email or in a contacts record. It was ultra-fast on the old BlackBerry; much slower on the new one.
It is nice that the Q10 solved many of the old BlackBerry's shortcomings, such as the basic strength of the operating system. The Q10 has a stable OS, and all the apps seem stable. It now can handle a key app such as Skype.
However, fixing many of the old BlackBerry's shortcomings is not a step forward if it also kills some of the old BlackBerry's key advantages. You have to be able to move at lightning speed to answer emails, edit text and handle the address book. The new Q10 has lost some of these key legacy BlackBerry advantages.