Streamingmedia.com has published an initial comparison of Google's VP8 open-source video codec (part of WebM) and H.264, run by Jan Ozer, who's done many such codec and compressor comparisons over the years. In summary, at almost exactly the same bitrate (438kbps for VP8 vs. 439kbps for H.264) and the same resolution (480 x 360), H.264 provided slightly better overall visual quality, especially in clips with higher motion. However, Ozer states that the difference won't be noticeable in most applications.
Keep in mind that the tests were done at SD resolution and at one bitrate, so more definitive testing will be required to understand how the two codecs compare over a range of conditions. However, at first glance, VP8 seems to be well in the ballpark with H.264.