I don't see anything wrong with the UI. I haven't experienced a problem with security. On Windows, I like that it provides Apple's font rendering; much neater for me. Here's an anti-list for you.
Chrome installs GoogleSoftwareUpdateAgent on Mac, which cannot be configured in any way, and can barely be uninstalled. Through Terminal which, while adequate for me, isn't adequate for the average computer user. I don't have broadband and I don't want Google deciding when they can fetch and unload 30mb packages on my machine (doing so when I'm on 125mb-capped cellphone connection is not a good time).
Firefox wasn't fast until 4.0. Plus I ran into a bug on Firefox 3.5 that blocks one key feature of a project of mine with its buggy sessionStorage implementation.
Opera is wonderful, but its Dragonfly is a remotely-hosted joke which I cannot use while offline or on slow connection. Plus, as of early 2011, I cannot continuously sync bookmarks with any other browser.
IE9 is IE.
I also like Safari's Speed Dial implementation (I like it better than Chrome's and Opera's). Nonetheless, I have all of the above on my Mac, except Chrome, due to its background malware being required for it to run. Malware because it actively hurts my wallet.