Why shouldn't you use such things? Here's just an example:
http://www.lifehacker.com.au/tips/2009/01/30/ google_chrome_accesses_hotmail_by_pretending_its_safari-2.ht ml
Websites often block certain browsers because they don't want to deal with cross-browser compatibility, so browsers will often spoof the userAgent/etc. to appear as another browser. userAgents are also subject to spyware which can severely corrupt them.
So after many headaches, I finally found a subtle difference between the two browsers that can be used for browser detection:
if (window.devicePixelRatio) //If WebKit browser
if (escape(navigator.javaEnabled.toString()) == 'function%20javaEnabled%28%29%20%7B%20%5Bnative%20code%5D%20%7D')
else if (escape(navigator.javaEnabled.toString()) != 'function%20javaEnabled%28%29%20%7B%20%5Bnative%20code%5D%20%7D')
This works by the fact that Function.toString() in Safari preserves whitespace while Chrome's does not. This is not likely to change unless Google considers it a bug; I'm thinking they did it on purpose, and I don't see Safari changing their ways.