Since I updated to Snow Leopard, my Address Book  has been more or less unusable for anything other than browsing. Starting up the app resulted in 30 to 90 seconds of spinning beachball before contacts would populate. That clearly fell into the "annoying-but-tolerable" camp, since I usually only open Address Book a few times a day, and quickly learned to just leave it open to avoid the wait. But adding new contacts, or editing existing ones resulted in the same spinning beachball of doom.

My first instincts were to export the data, zap my Address Book files and re-import, but the problem persisted. I forced an overwrite from MobileMe. Nothing. I shook my fist angrily and threatened my iMac with bodily harm. And the beachball continued to taunt me.

Finally, I called Apple's tech support. After multiple restarts, trashings of Address Book data, and forced MobileMe syncs we nailed down the culprit: Smart Groups. After deleting two Smart Groups from Address Book, the problem just disappeared. According to a helpful Apple support tech "We know that Smart Groups are not working with Snow Leopard right now. Hopefully we'll get it figured out in the next update."

Until then, delete them.