Loved the Paris Metro and RER trains. I went everywhere in Paris via RATP and used their online…read morewebsite to get around to various destinations. Also used Apple Maps with it. Are the stations and tunnels old & dingy? Sure. But they do their jobs. It reminded me of the subways in NYC because the trains were so frequent and accessed everywhere I wanted to go. Compared to San Francisco, it was amazing.
Just be aware at the Montmartre station, the stairway up goes round and round and round forever. It was never ending, but I was very grateful to not have been one of the many tourists there who were lugging their rolly bags up those never ending steps with them.
Some of the agents working in the Metro stations don't speak english, even in the main large stations. Luckily the little station I used for my main departure route had an english speaker and suggested I get a Navigo card since I was staying 2 weeks. It cost 10 euros, you need to get a photo (much smaller than US passport size, and they will cut it down to fit) and they will upload whichever amount pass you want on here - weekly pass is minimum and those are from Monday to Sunday, no matter which day you purchase it on. The Navigo card is good for ten years.
Just buy your tickets and keep them on you. The 2 weeks I was there, on three occasions, Metro Police/Officials checked everyone for their tickets. Two times was in the tunnels of I don't remember which stations, once was just before arriving to the Palace of Versailles when they fined a handful of confused tourists and an international american student 35 euros each. A ticket to the Palace of Versailles Rive Gauche station from the Saint-Placide station in the Louvre area was just 4.15 euro.
If you're disabled, RATP may be difficult to navigate. They do say they've made many stations disability friendly with escalators and elevators, but it didn't seem that way with many of the stations. They're still working on it. Paris is imo not a friendly place for disabled and wheelchair users. Alas, too old and too many cobblestones. Things that make the city amazing also has its drawbacks and this is one of them.