I'm a front-end web developer, so the first thing I do after restarting my Mac is to fire up Safari, Chrome and Firefox (I'll use IE and Opera mostly just for testing).
I use all three all day long. It helps me segregate my work; I might use Firefox and Firebug to preview and debug my work, Chrome for research, and Safari for another project. It really varies from day to day. None the less, I use all 3 a lot.
I really want to make Safari my primary general use browser, but there are a few things holding me back. As others have mentioned, tab management still needs work. Please give us favicons in the tabs, it makes it so much easier to find the right tab with several open tabs. I really miss pinned tabs, too. Again, when managing a ton of open tabs, that feature really helps.
Another Safari shortcoming - address bar handling. The latest version has improved, but I still have a problem actually clicking inside the URL address bar. I am also highly reliant on the 'paste and go' feature supported by both Firefox and Chrome. I spend a lot of time copying URLs from one source and pasting into the browser, so paste and go (and paste and search) is a really useful feature for me.
So like a lot of other people, Safari still falls a bit short on giving users the ability to make the browser 'just so'. I love the speed and lower CPU/Memory overhead of Safari, and some of the new developer tools look really helpful, but the lack of the features mentioned above cause just enough friction to keep Safari as a secondary browser for me.
Also, Chrome Canary 64-bit makes a huge difference on my MBP. 32-bit Chrome just killed the battery and hammered my CPU. Chrome Canary makes it easier for me to lean on Chrome as my #1 browser, but I'd switch to Safari if Apple improved support for extensions to allow 3rd parties to bring these features to Safari.