Just look at the PT numbers, they all did great at the box office.
TPM- 431 million
AOTC- 310 million
ROTS- 380 million
But how many non-SW fans went to see these movies? I have one friend who saw TPM 6 times in the theater in 1999, that is a big reason SW numbers for all the movies are so high, they get repeat business from the same fans, and more repeat business from the same fans.
Lord of the Rings got that too, when each of their movies ended up at roughly 350-380 million, that is the cap amongst true fans for all these movies. Spiderman did 407 million, and the sequel did alittle over 300 million. But again would recommend these movies to any movie goer?
A movie like Titanic did 600 million, and was a true phenomenon in 1997-98. Now granted many fans saw it numerous times that pumped up the box office, but I would still recommend Titanic for anyone to see, my brother, my sister in law, a teenager, or even my mom or grandmom.
When the original SW came out, everyone of those people I just mentioned saw it in 1977. Partly because they 'had' to see it to see what all the fuss was about, but overall it is a standalone movie that is an enjoyable, fun movie that has an ending and could have ended there. I would not hesitate to this day to recommend the original SW to anyone of any ages to watch it, and putting aside the 5 sequels, I am pretty sure they will enjoy the movie, and maybe even really like it.
For the other 5 SW movies, I don't know if I could recommend them. To me, as good as ESB is, and I think it is just as great as SW original, you still have to LOVE SW to like this movie, it delves more into the characters, it doesn't have an ending, and the good guys lose in the end. But it is atleast a great movie, so that would be the only sequel I would think to recommend.
ROTJ, TPM, AOTC, and ROTS are just sequels that live by the SW name, and other than SW fans, anyone non SW fan movie goer would think these movies are pure shit.
The original SW is one of those movies that struck lightning in a bottle, I can't explain it, but it is like E.T., Titanic, The Godfather, where the success lived up to the hype. That is why everytime it gets re-released in the movies it crushes the sequels, because you and I and most on this website went to see all 3 SE movies in 1997, but the mainstream audience went to the SW and thats it.