It is a bit dated since much of it relates to agonizing over Vietnam War draft dodging and there is just the beginning of open writing about gay relationships.
In general there is a lot of agonizing over trivialities among the characters in this book. I dislike books about people who make their lives difficult for no reason and then whine about it (see my review of JUDE THE OBSCURE). In AN ACCIDENTAL MAN many of the characters make their lives difficult for no apparent reason except that they are bored and overpriviledged--but thankfully they don't much whine about it.
There is not much plot although some odd, unexpected and violent events occur. There are obscure passages that reminded me of the worst of Henry James. And many passages could be skipped or skimmed. E.g. there are long series of letters back and forth and extended cocktail party conversation.
But I realized that the happily married couples lived their lives calmly in the background while their unattached siblings and children made themselves and others miserable. A great testament to ordinary middle class life (although I'm not sure that's what Iris intended).
Basically, I liked the book because in spite of the above I cared about the characters, got emotionally involved in their lives, and felt that I had been in touch with something interesting and important. The main difficulty that I had with Iris' writing is that she does not, at least in this novel, make any love relations comprehensible or believable. It's as though Iris does not know what love is or has never loved. Maybe however this an artistic aritfice and part of the "message" of the book. It just ain't true that "all you need is love." Mostly it's phony and unrewarding.