I don't think the PT failed strictly because of the turn. Personally i felt that the emotional angle of Anakin's turn-- an obsession to save a loved one from death which results in him selling his soul to protect Padme, only to have this very act kill her and thus leave him transformed into a monster--is the most brilliant stroke that Lucas came up with since he made Vader and Father Skywalker the same person in 1978. Yeah, the youngling slaughter right afterwards doesn't fit in at all, but overall i though the emotional motivation behind Anakin in Episode III--or at least for most of the film, since the consistency breaks down near the end--was the most compelling and believable part of the entire trilogy. But whether you thought the turn worked, failed, or sorta worked (i am in this category), the PT's fundamental flaws are entirely different--no six-film series could possibly be "ruined" by a mere sequence in one entry. The problem with the PT is that the films were simply poorly made right from the get-go. The writing ranged from acceptable to laughable, the performances were generally not believable, the plotting was ponderous and inconsistent, the directing embarassingly amaturish in most respects, and a lot of the action scenes not very exciting. The cumulative effect of all these things is that the characters were utterly hollow. I just didn't connect to or care about the characters because of all of these factors. Watching Anakin's fireplace love pledge in AOTC for example, or any scene between Obi Wan and him from that film, all i can do is laugh at how bad everything is done. TPM sorta got by because it came off as a self-contained fantasy picture that was light and not too important in the scheme of things, but once we get to the actual story in AOTC this superficiality simply cannot stand. The films were just shittily made, and ROTS didn't have much of a chance because of it. Personally i feel that the first forty or fifty minutes of ROTS is quite an impressive feat in that it partially succeeds in not only making the characters somewhat believable, if only in a minimal way, but that it almost undoes the damage inflicted by the previous films. Personally i find that ROTS works the strongest without the previous two films--you don't know that Anakin was a whiney baby, that Padme is a hollow robot, that Obi Wan and Anakin came off as hating each other, that Anakin and Palpatine had a non-existant relationship, that there was a complete lack of romance between Padme and Anakin, that the jedi were chumps, and that the films were overall dull and uninteresting. With all these things weighing it down, it is suprising that ROTS works in any way.
These same flaws are the same things that also ruin ROTS--inconsistent characters and characterisation, weak dialog and unintelligent plotting; these manifests themselves in the turn scene. Luckily, they were balanced out the first dose of suspence, interesting story development and compelling characterisation that the prequel films had seen, such that many people thought the film worked in spite of the flaws. The first half of ROTS was well done to such an extent that when the film finally returned to the inconsistent levels seen in AOTC it was all the more jarring, especially in the climax where it alternated back forth between them with rapid pace (Anakin and Obi Wan fighting on Mustafar--cool; Emperor becoming a cartoon character--what the hell?; Anakin burning and being rebuilt as Vader--cool; Padme loosing the will to live--what the hell?).