I've found Address Book's 2-panel layout in Lion extremely obnoxious because I must change modes to change groups, never remember what group I'm viewing, etc. Is there any way to restore the old 3-panel layout? If not, recommend any open source alternatives with sane layouts?
I don't have my SL disk handy, so I cant vouch for this, but maybe try copying the Address Book.app from SL (or a backup) over to Lion?
Take a look at "Addresses" an alternative address book application:
It is not open source, but is is just $3.99