I'm certainly no expert but I played for several years without a GPS or range finder and actually never looked for yardage markers very often. I just went by what I saw.
I eventually got a GPS and now I'm spoiled and really don't want to play without it. I'm not sure how many strokes a round it saves me but I'm sure it's at least a few. It not only gives me a better chance at choosing the right club but without a doubt I make a more confident swing and trust it more.
I've thought about getting a range finder but so far I haven't seen an advantage to it over my GPS. Some of my playing partners have range finders and they usually come up with the same distance that I do (give or take a few yards occasionally) and if there is no line of sight they often ask me what I've got.