In their respective times they were a big improvement. I believe CVS was the first client/server revision control system (why that feature was added was a horror story)
Which version control you use matters a lot less than having a sane development process.
(RCS only worked on single files, moving to CVS allowed you to maintain a tree. Before RCS I was using SCCS!)
Which version control you use matters a lot less than having a sane development process.