I've never walked out of a movie, including the prequels. However, I have given up watching a movie on DVD because (one of the Nolan Batman films - it was awful, and about 1.5 hours too long). I'd probably have walked out of the cinema if I had been watching it there (probably not when I was younger, but I have a much lower tolerance of super-serious yet inane crap these days; also, my time is worth a lot more than my money - when I was younger, it was the other way around).

If the sequels turn out to be as bad as the prequels, I'll certainly walk out (after giving them a decent chance) - I'm not going to kid myself they must be good like I did with TPM. This is partly for the sake of anyone I'm with - I won't be able to resist quietly dissing them if I stay in the cinema, and that's not fair to anyone I'm with.