I love Sparrow for its simplicity.
However there is one problem that makes me pull my hair out. As you receive mails from various sources, Sparrow slowly overrides the OSX Address Book contacts with the format your senders have used.
However you can reset the contacts list easily by deleting the index file email.plist.
Quit Sparrow, go to Terminal and type:
cd "~/Library/Containers/com.sparrowmailapp.sparrow/Data/Library/Application Support/Sparrow" mv email.plist email.plist.bak
This will rename the index file as email.plist.bak, and once you restart Sparrow, it will once again use the contacts from your Address Book.