A word of advice to anyone setting out to read this book for the first time: Be patient. It take a long time for the story to really get going. Yes, several of the characters are fascinating from the get go, especially Jenny Fields, Roberta and even poor Tech Sgt Garp, but it's only John Irving's wonderful writing style that prevented me from putting the book aside. I found it exasperating at times, especially Garp's silly Grillparzer story that struck me as a literary equivalent of contemporary "performance art". The episode between the Garps and the Fletchers comes perilously close to the kind of musical beds nonsense made popular by John Updike (and that would be a major downgrade). Even the notorious driveway episode that everyone remembers about the book seems contrived, requiring more suspension of disbelief than I can muster. After having read the glorious "Ciderhouse Rules", this book was shaping up as a major disappointment to me.
The real turning point comes later, when all the deaths suddenly occur. That's when John Irving grabs you and you're in for an emotional ride. As in his other works, Irving constructs a host of damaged, vulnerable, flawed characters and causes you to grow to love them and care very much about what happens to them. Almost every one of them turns out to have redeeming qualities, and over time, they are able to change. Garp makes much of the value of writing what is "true" about the human condition and his ability to shape the trajectory of a character's personality over time is a perfect example of that "true". In real life, people are not static, but very few writers are able to accomplish that kind of life journey for their characters. Not since Vitor Hugo have I encountered a writer able to do it as Irving does.
There's definitely a "Spiegel im Spiegel" aspect within the book: Irving portrays Garp as a writer who starts out with pure fictional inventiveness (writing about "the world" as he terms it) but regresses into layers of autobiography. Garp is clearly a portrayal of much that is just John Irving, partly as he is and partly as he wishes to be. So you need to like Irving in order to like Garp. Fortunately for me, I like Irving, so I found Garp to be tolerable. It's interesting to note, though that while reading the epilogue, I didn't really miss Garp that much, but found myself mourning the loss of Roberta, Helen, Ellen and even the memory of the indomitable Jenny Fields.