There is some interesting data you can extrapolate from sequel sales, for sure. FFX sold 8 million copies, and 5 million out of those 8 million customers thought FFX-2 was worth getting. If they had hated the first game, I would assume they would not be interested in the sequel.
FF13 sold about 7.5 million. FF13-2 sold less than 3 million. LR of course, sold less than 1.5M.
What this tells me is that a lot more of those who bought FFX enhjoyed their experience compared to those who bought FFXIII.
To compare: Mass Effect sold 3.5 million copies. Mass Effect 2 sold 3.5 million copies across the two platforms ME1 was released on, then an additional 1.5 million on PS3.
edit: to clarify, these statistics do not include re-releases and trilogy collections, such as the ME Trilogy pack for PS3, or the FFX/X-2 re-release on PS3 and Vita.