The Marlins have been very consistent in quality in my experience. They have made some changes to stock designs over the years. They offered some budget versions with cheaper wood and sights, but the top of the line 336C has always been well made. Even the budget models are well made, just using cheaper wood and less polish on the metal.
I have heard some grumblings about some very recent guns not being quite as good, but have not experienced any issues personally.
To me the 1970's Marlins are the nicest looking. Current guns have a fat foreend that just does not look right to me. Many from the 60's and earlier are even fatter than current production guns. During the 70's the foreend was smaller and trimmer, much like the Winchester 94. That was also before the lawyers made them put a safety on the receiver.