The 30-30 cartridge is an excellent, all-around, general purpose cartridge. It will never equal the ballistics of a .308 Winchester but never let the "nay-saying naybobs" tell you the that the 30-30 is inferior.
It is a poor choice for the deliberate hunting of dangerous animals like coastal brown bears, but easily dispatches garbage raiding blackies, younger, cabin wreaking grizzlies, and chicken-stealing foxes.
As far as the differences between the JM stamped rifles and the ones made after Remington purchased Marlin, well, there were some quality issues with the first Remington rifles (I had to send a .22 back for a correction myself) but I think those quality issues have been resolved by now. My favorite Marlin is my Reming made, youth model carbine. It is accurate, super portable and so easy to swing. I have plenty of firepower available with that rifle. Still, if I see a Marlin at a gun show I will look for the JM stamp but for me, I think that is just a habit.