Right off the bat there are several things that I like. Obviously it integrates with address book and iCal. I've only played a bit with the Address Book part so far.
It allows me to add fields to my address book contacts that are heretofore impossible to create in Address Book. And it is very easy to do so. This allows me to do what I'd like to do in Address Book, but cannot do, which is simply create a smart album which says that the Board of Directors (via a field) for Organization A is ____ and to do so very easily.
However, I do not see a way to reverse this where I can see the new fields I've created, or "Collections" (Address Books groups) in Address Book itself. This would be ideal. But in either case, having the ability to ramp up the info on my address book contacts with very little effort is a really useful tool.
Update: So, this does potentially work really well. I do a lot of media work. So I have a media group in Address Book. In Benta I just deleted all of the names. I looked into address book and the group was empty. I then created a field that is a checkbox for media. Went through and start just checking it next to names. Did a sort by media, dumped them back in the "Collection" that was imported from Address Book. And they magically reappear in Address Book. Very easy (could be easier) and very helpful.
So here is my plan. Use Benta to create checkboxes for all of the groups I work for. Create a custom form that shows the name, company, and just the checkboxes. Now I can see for my address book people if I remembered to add them to a group. Now I just need to automate the updating. But I cannot figure out how to get a smartcollection created in Benta into Address Book. I've been looking for these features in an easy, intuitive, drag & drop environment for a LONG TIME!