I'd love to have the capability to browse my 400+ listing Address Book by Group. I'm positive Apple will get to this, but for now this online address book is way to early a version for me to think seriously about using it.
Other bugs -- Sync cannot be optimized for one feature, i.e. Calendar only, or Address Book only. I might not have a need to update calendars until once a week or so, or vic versa.
For those of us who are paying for .Mac now, this is more like beta software than official release software. I'm sure it will get better over time, but the value of everything (right now) seems pretty weak. I'm not complaining, but in a couple years we're going to look back and say "I paid $99 for THAT?!"...