Of the tests that are floating around the internet, WebM is comparable to H264 base or main, but not high (the different profiles are almost like different codecs, requiring more features as you get higher). Considering pretty much no phones can play high, or maybe even main, the quality comparison is kind of moot, unless you roll multiple versions of the video for different devices.
But no, the one of the advantages of WebM is that it is patent free (there is always a chance of submarine patents though, same with theora, although since VP8 was originally made by a company it is hoped that the patent pool is complete). The other advantage is that it doesn't cost money to use it. Currently it costs a few million dollars per year to buy the rights to distribute a program with the h264 codec, if you look at mozilla for example, this is a significant fraction of the money they raise each year. Currently anybody is free to distribute h264 video, but MPEG-LA could change this at anytime (although, it would be a very bad move if they did). So you have a codec is irrevocably free, or something that requries money to distribute codecs of and could cost money to distribute videos in.