5 GB a month is extremely low usage; that might be okay for my mom.
Netflix Instant is already used by millions of people (on PC, 360, and other devices) who aren't getting hit with crazy bandwidth fees. And the video quality is scaled from YouTube-levels up to 720p or so, depending on your connection speed. The service auto-selects the quality so that your buffer can keep up with the movie. It's not perfect, though -- I'm always getting a message that my "connection has slowed", and I have to wait a minute while they rebuffer a lower quality stream.