1Password 4 is a big leap for me in terms of usability (vs. v.3): the mini application can be quickly accessed via a key chord, similarly to what you can do within a browser. Very helpful for things like VPN access, encrypted HDs, and other non-web softwares.
The security audit is a great feature too: it can tell you where you are using weak passwords, or even repeated passwords. If you have used the same password on many sites, and one of those sites is compromised, you might want to change that password elsewhere, or better, make sure you use a unique strong password everywhere; so knowing what sites could be compromised is a huge help.
If you want to feel safe in the web, 1Password's high usability and new features will help you get there.