Surely for images served over the web the key measurement is the speed gain due to smaller filesize minus the loss due to extra decompression time. It is after all a project hosted under Google's speed project.
Opera certainly seem to think they can compress and decompress the WebP image on-the-fly and still come out ahead speed-wise compared with the original (at a cost of quality) and their old JPEG system (with improved quality), at least over low-bandwith connections.
http://code.google.com/speed/webp/
Which elsewhere claims that 99 human years are wasted due to uncompressed web content each day.
http://code.google.com/speed/articles/use-compression.html
Opera certainly seem to think they can compress and decompress the WebP image on-the-fly and still come out ahead speed-wise compared with the original (at a cost of quality) and their old JPEG system (with improved quality), at least over low-bandwith connections.