My biggest issue with Sparrow is failure to support "Unread" count across a set of labels. Those with a high volume of mail often set up filters to pre-file that mail. I then use Apple Mail's "Smart Mailbox" to roll up the unread messages from only those labels that matter, while ignoring the labels containing mostly bulk content.

For me, any program that interacts with Gmail and doesn't support seeing unread count across labels, can't be used.

Disappointed. I paid for Sparrow, and wish it could be used. The update wasting more screen real estate to let you choose between Inbox and another label could so easily have been the killer feature it failed to be.