Usually works really good. Might break some pages (like YouTube), but it's easy to disable it temporarily if it does. I have one problem with it though, the settings for what browser to spoof isn't saved. I clean out everything when exiting the browser, and somehow this revert the changes back to defaults (to Chrome). Not sure how the settings are saved, but they are lost every time i exit.

Also, an option to change the agent on every reload (instead of just after X mins) would be nice.

[EDIT] Got the settings to be saved by disabling Sync extension settings between browsers. Just a tip if other experience the save problem. The error occurs if you also purging the browser data on exit.