favorite for me. I was caught off guard by it and loved it almost against my will, despite the epistolary-ness of it.
The nature of my damage with epistolary novels is that I think it's disingenuous to have the narrator(s) scribbling away for chapters at a time - often with old-fashioned pen and paper - when in real life, if they did so, they wouldn't have time to actually experience any of the plot. You know, because they'd be writing all day. Sorcery and Cecelia is a fairly egregious offender in that regard.
Another issue is that I find epistolary novels to be singularly disorienting. It takes me much longer to wrap my head around the principal characters and plot of an epistolary novel because the point of view jumps around so much and the reader is thrust right into the middle of the action. I find myself having to pay meticulous attention to tiny details, often flipping back pages to answer questions such as, "wait, who was the letter to? When was it written? And from where?" It can get a little tiresome.
The final off-putting trait of epistolary novels is that in general, you know the characters writing the letters emerge from any and all danger at least relatively unscathed. They're writing the story, after all. Even if a character starts off a letter saying, "Oh, sister, let me tell you all the terrifying and amazing events that have come to pass since I last wrote!" the suspense is killed because, well, she's writing the letter so she's fine. I remember The Woman in White being especially ridiculous in this regard, but it's only partly epistolary so I forgive it.
All that said, if an epistolary novel is good enough that I'm willing to suspend disbelief for a while and accept that it really didn't take that long to write all that stuff down, then I'm all for it. As obviously was the case with Guernsey, and, to a slightly lesser extent, Sorcery and Cecilia (which, FYI, I think you should read. It was described to me as a cross between Jane Austen and Harry Potter and that is pretty accurate).
Have you ever thought to form an opinion on epistolary novels? What are the worst offenders of this genre? What are the best examples of it?