I'm a pretty big movie buff, so the decision to switch/adopt for me will probably come when I can walk in to a Blockbuster and see not just a separate shelf, but a separate section on hi-def movies.
Since I'm going to have to get a (new) 1080p TV anyway when the feds stop all analog broadcast TV signals, I'm thinking it's all just a matter of time. By then, 1080p TV's, dvd burners and blank discs should come down in price considerably to make the purchase more acceptable for my budget. The format war should also hopefully play itself out by then too.
As a PC user, the backup capacity will definitely interest me, as long as it doesn't take longer to fill a disc than the regular DVD burn times are now (yes, I know, current models are FAR from that). Software that lets me create hi-def movies and directly copy video from hi-def camcorders is also something I would want.
For right now though, I am happy with regular DVD. It's not a question of "if" I jump onto the HD bandwagon but rather just "when".