They offer a trial iirc that lets you get an idea of what it has. I wouldn't say it really has a single killer feature, but it adds a lot of little things that together make it a chore to not have once you get used to it. A couple of common things I us:
1. auto adds whatever prefix you want to fields/searches/etc
2. switch roles but land on the same page
3. formula auto complete (extremely useful)
4. sticky columns
5. split view saved searches
6. live view on searches and various pages