I would say it is a good idea for those on dial-up (or slow broadband), some sites are so media rich as to be very slow loading and the user ends up passing it by (a loss for the site).
One thing I'm finding now having just got slow broadband, if watching a flash video clip, the default is to display it at the highest resolution it is very, very, slow. I end up right clicking and lowering the quality, but that seems to have little effect (doesn't seem to impact that clip as it is already being downloaded) and the setting isn't remembered or retained.
So if this new webp format retains the user preference it would save a lot of user input. Whilst it doesn't effect existing image formats and video clips, it may persuade site designers to use this type of image format rather than lose a potential customer.
