Matthias,
I've just purchased Addresses from the Mac App Store and have posted a review of the app.
As I am sure you have gathered, pretty much everyone would like the new Apple Address Book for Lion to revert to the version used in Leopard and Snow Leopard.
For me having three panes is essential, having Groups is essential, having the ability to quickly see who is in which group would be great and, possibly most important of all, having the ability to Edit the Distribution List for all those people who seem to have email addresses for home, work and play, but you only want to send a group email to one address.
Apple Address Book had this facility in OS X 10.5 and 10.6 but it seems that a couple of children have been let loose on the development of this Lion version.
At present Addresses is not up to the mark, sadly, but if you can make Addresses just match Address Book in OS X10.5/10.6 you will have a winner on your hands! Keep up the good work.