Certainly, there's a lot of really atrocious content on YouTube, but there's also a lot of really interesting, informative, entertaining stuff as well. If you pick carefully, and as always, apply some critical thought, you'll find a lot of good things to watch.
Absolutely. Theodore Sturgeon must have had a time machine when he said "90% of everything is crap". He must have traveled forward in time and watched Youtube. (My corollary: "Sturgeon was an irrepressible optimist.")
But those few gems make it all worthwhile.
Some of my favorite channels include anything by Brady Haran (periodic videos, sixty symbols, numberphile, nottinghamscience, computerphile, etc). Smarter Every Day; TheRoyalInstitution; and Scott Manley. Scott is one of the very few Youtube space fans who really seems to know his stuff.
And in the non-technical areas, there's College Humor. Like all humor it can be very uneven, but they have some absolute gems like "Why Every New Macbook Needs A Different Goddamn Charger" and their "If Google Were a Guy" series. Hysterically funny.
Oh, and this one: