Unfortunately, WebM isn't _really_ a serious alternative. It's a fine codec and produces great quality video in a similar filesize to H.264, but it's not supported on the majority of mobile devices. H.264 is also what many consumer video cameras produce, like iPhones and most DSLRs.
Pragmatically, I'm happy that all the major browsers (with the exception of Opera) will now support one common codec, even if it's patent-encumbered.
Depends on the threshold for calling something "serious". It's far more serious than any alternative before WebM, that's for sure. No competing format will ever be an instant drop in replacement for an entrenched format, but there is a continuum of scenarios below that.
Pragmatically, I'm happy that all the major browsers (with the exception of Opera) will now support one common codec, even if it's patent-encumbered.