AMGEN is coveted no more than the SURF program at MAYO, John's Hopkin's SURE, Harvard's SHURP, etc. But AMGEN Scholars is the way you want to do do research at Stanford, Caltech, UCSF, Washington-Seattle etc, or one of the other 10 host sites.
So to answer your question, AMGEN isn't valued higher than any other program, it allows one to do summer research at a top research institution at a location that they would also like to maybe attend for med school or grad school in the future, maybe even be a place that they always wanted to visit and explore ( I always wanted to see San Francisco!).
But come on now, summer research at one of the premier (Stanford is 1st!) medical research institutions sponsored by a gigantic billion dollar biotechnology corporation in AMGEN, sounds pretty prestigious to me! I have no doubt whoever gets interviewed at any med school will know their summer research for AMGEN is gonna be an interview topic.