I also signed up for the 1-DVD plan just for access to the streaming library via my Tivo HD. When it works, it looks pretty good... but maybe 20% of the titles I've tried to watch have *horrible* problems. I don't mean "zomg it's not HD" sort of problems, but things like audio and video being out of sync by multiple seconds, or the video looks like an analog-scrambled premium channel.
For DVDs they have a "Report a Problem" button right in your queue and respond very quickly to any issue, but streaming video seems to be a best-effort, take-it-or-leave-it sort of proposition. If the quality were a little more consistent maybe I'd consider dropping the DVDs by mail, even with the current limited streaming selection.