I lived on the Upper East Side of Manhattan for five years in the mid-90's. This neighborhood extends from 59th Street to 96th street and the East River to Fifth Avenue-Central Park, officially. When I resided on the Upper East Side Neighborhood it was felt that you didn't really live in the area until 75th Street or thereabouts.
There's a perceived attitude about the area best summed up in a quote from the sitcom, Seinfeld. Kramer says to Jerry "If you're not gonna be a part of a civil society, then just get in your car and drive on over to the East Side". But when I lived there, just out of college, it was youthful, fun and full of things to do. Many things haven't changed in the neighborhood. There is the world famous restaurant Elaine's and plenty of other great places to eat, maybe just not as famous. Personally, I still love Cafe Trevi, a very romantic restaurant (which I can attest to via some great first dates!) with great northern Italian fare. There's also Daniel's, one of the consistently top rated restaurants in the city and run by the famous chef Daniel Boulud. What this translates into is you can spend hours celebrity watching.
There's a great cultural life, too. The area has enough great museums, modern (the Guggenheim is one), traditional (the Metropolitan Museum of Art is world reknown) and scientific (for example, the Rose, formerly the Haydn Planetarium) and the Museum of Natural History, (which everyone knows about - even if they've only seen it in movies) to keep any curious mind entertained. There's varied shopping, from the Gap on 86th Street to the exlusive Ralph Lauren Mansion on 867 Madison Ave between 71st St & 70th Streets. Not to mention the world's greatest respite from city living - Central Park.
It's definitely grown more expensive, however. On average, one square foot costs over $11,000. This means a very homogenous population.