WCM tip #23: Use the Vary: User-Agent response header when using Device Channels
Device Channels allow you to optimize your website for different devices. By using different Master Pages and Device Channel Panels you can control what is rendered for which devices and optimize performance of your website.
Rendering different HTML to different users might be considered as cloaking by Internet search engines and when it will, it might have negative impact on the ranking of your pages in search results. To ensure that this will not happen, you should use the Vary: User-Agent response header to let Internet search engines know that the rendered HTML might be different depending on the device used to open the web page.
One downside of using the Vary: User-Agent response header is, that with the magnitude of different devices and browsers, many proxy servers treat this header as Vary: * which will basically prevent your website from being cached on those servers and might have negative impact on the performance of your website after all.