I have tried Mercury, Atomic and chrome and am still not clear of using the standard mobile Safari.
There appears to be no clear choice for an alternative browser.
For me, it's the bookmark syncing. So far as I can tell, and the info that came out with the release of Chrome for iOS confirms it, nothing on iOS will be faster or render better than Safari. So if it wasn't for the fact that there is no way to sync my desktop Firefox bookmarks to Safari, I'd be using that.
If you are having a hard time getting "clear" of using Safari because links from other apps open Safari instead, there are a few Jailbreak tweaks that can set your browser of choice as the default so it'll open for outside links instead.