I agree as well that I think Word of Mouth is good for ROTS... Reviews were predominantly positive (negative reviews I've found often were stretching for things to complain about), and on a personal level my friends who aren't die-hard SW fans were floored by the film and consider it as good as the originals... I think in general that people loved ROTS if they went to see it.
The built-in-audience is a given... I stick by my initial thought that the lackluster performance of TPM and AOTC has hurt ROTS in the long run. When Star Wars was re-released back in 1997, it was cool to go see it... It was cool to like Star Wars at the time really... TPM drew a similar crowd then, but people didn't like the film. It had that bigger draw because more people went to it but in the end they really didn't like it... AOTC did piss poor (relatively) then, but was a better film by most everyone who saw it's standards. THe franchise, after TPM, had become like the plague for a large portion of people it seemed... It was nerdy to like Star Wars because "oh, so you must like Jar Jar Binks!" and other asinine remarks.
Jump to ROTS and it still suffers the "this is dumb... I bet Jar Jar's in it!" Stigma. I personally think that's made a lot of people who (if the franchise wasn't so stereotyped as "bad" now) otherwise would've gone to see the Star Wars films say that they'll either wait to rent it or they're done with the franchise overall... The disenchanted. That's just my opinion though... To me, the film got lots of good press and talk from the public, but that's just my personal experiences with it too.