lucy24 - 10:52 pm on Mar 8, 2013 (gmt 0)
The glory of the "max-width" format is that it doesn't measure device-width ... except in those devices, currently including almost all mobiles, where there is no separate window. In a browser, it goes by the size of your current window, and changes dynamically. And that is very much what you want. Or, ahem, should want.
Speaking as a user, what I definitely DON'T want is a site that says "Hah! I see you have a 1600px monitor, so I'll serve you the 1600px version of my site." What I want instead is "I see your window is 800px wide, so I'll give you the version that fits that size."
My initial point was that you may not need to make a separate mobile site at all. It depends of course on content, but many times you can do it all with simple changes in stylesheet.
The overwhelming problem with user-agent detection is that there are simply so ### many of them. And sometimes a site makes the wrong choice, for example sending an iPad to the version made for iPhones.