Reston, on a sunny day in the late spring of its 20th year, is beautiful, almost paradisiacal.

One notices little things: The orange rims on the backboards at the basketball courts tucked here and there sport rope nets of the kind that shooters, when they're on, love. The ropes go "rrrrp!" (not "swish") when the ball pops through.

And there are, of course, the big things, most of all the trees, trees, trees that tower over curving roadways in the gentle Virginia hills, over miles and miles of bike, jogging and walking paths, over sidewalks, streams and lake fronts, over front and back yards of detached houses, row houses and garden apartments, and even, in incipient form, over a few of the more attractive macadam parking lots in Reston's burgeoning 1,000-acre industrial strip.

Twenty years after the first family moved into the planned community (in December 1964), 24 years after its founder, Robert E. Simon Jr., purchased the 6,750-acre tract in then far-out Fairfax County (and named it after his initials), and 23 years after the county government made the then epochal decision to approve, with certain reservations, an innovative residential planned community ordinance for the "new town," Reston and Restonians deserve their yearlong celebration of themselves. There is trouble in paradise -- there is always trouble in paradise -- but Reston has been a success in many, many ways.

The most apparent evidence is the beauty of the place, especially in comparison with conventional suburban developments. This physical attractiveness is less, much less, a matter of architecture than it is a tribute to planning.

Reston has its standout architectural features, to be sure, but it also contains many an architectural mediocrity -- one of which, unfortunately, also is the town's most visible structure, the 15-story Reston International Center, completed as a "seed structure" in 1972, and another of which, depressingly, is the recently completed South Lakes Village Center. Basically, however, the varieties of good, bad and in-between architecture are subsumed under the structure of a plan that respects the land and, in so doing, accommodates a mix of uses and building types that was unusual in a suburb of 20 years ago and, in large measure, still is.

The Reston plan has undergone quite a few important changes over the years. James Rossant, one of the architects who helped to draw up the original plan (and who also designed the sculpted Lake Anne Village Center, with its striking combination of high and low buildings that for many years provided the outside world with its primary image of Reston), complained at a recent symposium that "there has been no continuation of the structure we were attempting."

In a way he was right. The village centers (five instead of the seven originally foreseen) were to be connected by "street-like, high-density sinews" of development. Instead, the connective tissue consists of row house or low-rise apartment structures arranged on curving, suburban-type streets. In image and in reality, Reston today has a much more suburban character than Simon, Rossant and other pioneers envisioned.

There are minuses and pluses in this difference. One cannot help but feel that a great long-term architectural opportunity, a city builder's dream, was lost and with it the chance for a more heterogeneous mix of class and ethnic groups than prevails in today's Reston. At the same time one has to ask, in view of the impractical and grandiose kinds of megastructure that architects were thinking about 15 or so years ago, whether the results would have in any way lived up to the dream.

Then, too, one simply cannot ignore the economic realities of the Reston story. Simon was ousted in 1967 and his giant corporate successors -- first Gulf and then Mobil, the parent company of today's Reston Land Corp. -- understandably focused more on what was marketable than what was ideal. One of the amazing facets of the story is that Reston, unlike many a failed government-supported new town, was built almost entirely with private capital.

Given this fact, the degree to which the original land use plan was maintained is remarkable and praiseworthy. The primary difference between the first plan and the current one is mainly a matter of density, not principle. Reston's public spaces, integral parts of the plan, are tremendous amenities. Though the concepts of mixed use, natural beauty and public open spaces have been imitated in less ambitious suburbs, rarely have they attained the same level of cohesiveness.

"Reston: New Town or Suburb?" was one of the questions asked at a the anniversary symposium. The answers were two.

"The question is almost an insult," said Carlos Campbell, author of a book on new towns. "Reston is a unique living experience and it is not a suburb."

"Reston is a suburb," said Sylvia Fava, a New York City sociologist, "albeit a more diverse one than most suburbs in its metropolitan area."

Campbell was speaking with the passion of a longtime Reston resident. Fava's point of view was that of the dispassionate observer of statistical trends.

Maybe they were both right. Perhaps a new word is needed to describe the kind of mixed-density suburb Reston is becoming -- a place where increasing numbers of people live and work (about 40 percent of working Restonians are employed there) and where there are now more jobs than households. These trends doubtless will increase with the construction, later in this decade, of the long-planned Reston town center, a mixture of high-rise office buildings, residences and retail facilities.

But Fava was more right. Her statistics persuasively demonstrated that Reston, although its population of some 40,000 is a bit more diverse in income and race than the rest of Fairfax County, is true to the national norm of suburbs: It is richer, whiter and has far fewer social problems than the nearby city. This is not exactly news, and it's nothing to be held specifically against Reston, but if a picture-perfect slide show at the Reston Visitors Center is an indication, the town's original goal of exceptional diversity is on the wane.

Tom Grubisich, a Reston resident, journalist and author, with Peter McCandless, of the recently published book "Reston: The First Twenty Years," says he strongly resents the image of Reston as a "well-intentioned failure."

Reston is not a failure. By most measures it's a great success. But it also is a part of a regional planning problem of great significance. Although it is an economic and ecol- ogical whole, the Washington metropolitan region, like most nationally, is split into warring economic and political camps. Entry-level jobs go wanting in booming Reston, as in the booming county of which it is a part, while in the city teen-agers want jobs they can't find.

One thinks again of details: On city playgrounds where basketball is played, most of the rims don't have nets.