True & False
True: Because a lot of marijuana that was available in the 1960's was laced with LSD, Ketamine, Psilocybin, PCP, and other hallucinogens, the high from 1960's and today's pot is about the same.
False: Some pot that is available today has been recorded as having 8x more THC in it than what was grown in the 1960's, though these are usually hybrid plants (i.e. human cross-bred varieties). Additionally, today there is greater knowledge of more and stronger varieties of the marijuana than in the 60's, which can also influence why some people say today's marijuana is 'stronger'.