I've seen one or two of these a year for the last three or four years. I agree with everything TB said -- they're extremely well done. You can completely forget that you're watching a screen in a multiplex. I think attendance varies in different areas, but you can order tickets online:

http://www.metoperafamily.org/metopera/liveinhd/LiveinHD.aspx

NPR is still doing its Saturday radio broadcasts from the Met.