I have, and it works well.
While connected to the MHL adapter, the phone enters "dock" mode, which allows the home screen to be displayed in landscape orientation, both on the phone's display and on the external display. This matches the usual orientation of a TV or other monitor, but unfortunately some apps are locked into portrait orientation, including the movie selection interface of Netflix. Once you've chosen a movie in Netflix, however, it will play in landscape.
The output resolution is the same as the phone display: 1280x720. A 1080p TV will generally scale this to fill the screen automatically (though not correctly, keep reading). Some 1080p (or higher) monitors intended for use with computers may cause the mirrored display image to be surrounded by a black border. This can be adjusted via the stretch/scale settings of the monitor. On a 16:10 display--for example, 1920x1200--stretching to fit the entire screen will cause some distortion, with things that ought to be square appearing taller than they are wide. A 1280x720 or 1920x1080 monitor will display the image with no such spatial distortion.
On an HDTV, a black border isn't as likely a problem. However, TVs often zoom in the image from an HDMI input, beyond what is actually required to fit the image to the screen. This causes the edges of the image to be clipped. While this can be useful for television programming--HD broadcasts with content near the edges are actually formatted with the expectation that displays will do this--it is less than ideal for mirroring another display pixel for pixel. On my Samsung TV, the setting to disable this undesirable zooming is in the menu under Picture->Size->Just Scan.