I love my 66. Mostly because of how light and handy they are. The loved or hated plastic stock has a very nice shape which lends itself to be a very natural pointing rifle. As for the cyclic rate? Shoots as fast as I can pull the trigger...same as other semi autos. I don't think the design allows an industry to be built around accurization and customization, like the 10/22...you can find different color combos of 66's

. My guess is that the 10/22 has more intrinsic accuracy potential than the 66. The barrel and reciever(cover) has more of a flexible attachment on the 66, more solid on the steel, aluminum, and wood of the 10/22. The plastic trigger is a bit mushy and the lock time seems long. If you are a neat freak and need access to every crevice of the rifle for cleaning each time you shoot the rifle, the 66 is not for you. I take mine apart less often than other gun designs, clean the parts I can access, and shake out the unburned powder from the remaining areas and it functions great.
My guess as to why this design is no longer made may be legal considerations. The earlier 66's did not have serial numbers(as with many other .22's of that period). When legislation called for serialization, the steel reciever covers got the numbers. Now you can take off the reciever cover, re-insert the bolt handle, tape down the ejector, then you have a functional .22 semi rifle without a serial number(and without a rear sight). I might be reaching a bit, but legal counsel types might have gotten nervous.