It is only 4 shows into the series, and I do believe this show is both, for real and going to last.
ABC picked up the full year, which really was not a surprise, but the shows are just packed with content and nuance to captivate even the most wayward of skeptics. There are so many ways the producers can take any one of these families, the available content should be able to last as long as the writers can keep up. So far at least, the writers are doing just fine, and the cast is right on top of giving us the feeling that we could be in their reactive roles.
There are so many relate-able scenes, the type where "we've been there", that a person gets drawn into the scene and anticipates the characters reaction based on their own experience.
There also seems to be a steady stream of Hollywood notables who are offering their services up for "spots" to be written for them. That in itself says a lot, and the writers are putting them in roles that fit like a glove, which gives the show just that much more depth.
Modern Family is a "Gem" of a show, and certainly gives me hope for a resurgence in network programming.