I used 66's back in the 70's -80's. They were the best thing going at that time. I don't recall any issues with them. Later I started using DR nickels. Then about 4 years ago, I put some 66's on my Jazz. They're still on it and they sound great. A little rough on the fingertips and tight on the string tension on the D and G. They still have that slight "grind" to them. Great for rock and slap.