This is like a dicey answer and hence the silence I guess.
Since you explicityly mention Calendaring and addressbook and you were using Palm before, I woild be hesitant to reply to you.
But heres the low down- PIM wise the zaurus is not there yet, at least not as far as the Palms. For someone like me who never used a PLAM the Zaurus PIM\'s (with third part PIM\'s too) just doest it job perfectly. And I dont sync either. My Zaurus is my primary PIM storage device.
You must have read of frustration with the Zaurus\'s PIM capabilities, so no use saying that the Zaurus is the best. It is NOT.
As for other usages and hardware wise - the Zaurus is the BEST.
So there you go, kind of an ambigous answer.
Now if the real power of the Zaurus as a handheld computer attracts you, then you might consider the Cxx series (C700, C750, C860) over the SL-5xx series (SL-5500, SL-5600). This is because the screen on the C series are full VGA screens and are the best in the whole handheld segment as a whole.
For availability - the C series are not introduced in the US (I dont know from where you are), but is easily available from re-sellers like Dynamism, Conics etc. The SL-5xxx series have been taken off from the retail market like bestbuy etc but available online from Amazon etc.