How robust is the mobile detection? If using the user agent to determine a mobile browser results in many false negatives then not directing from the mobile si

How robust is the mobile detection? If using the user agent to determine a mobile browser results in many false negatives then not directing from the mobile site version is a good idea. Edit: To be more clear. If you are on a mobile device and you are trying to access the mobile site but the server can't reliable determine that you are in fact on a mobile device it would be bad to auto-redirect. At the same time accessing the mobile site on desktop browser should still allow the site to be usable.