I've used Knockout for two applications and it's great; MVVM is definitely more relevant for JS web applications than MVC in many, tho not all, cases.
I haven't done much GUI programming, so I'm not familiar with the classic definition of MVC. KnockoutJS's MVVM (really V-VM) is very clear and I love it.
KnockoutJS would benefit from the addition of Models (in order to make it a 'framework'), but I'm 90% more productive right now and am certain that KnockoutJS will evolve to include Models.