THE EAGERLY anticipated ''Hook,'' Steven Spielberg's updating of the J. M. Barrie classic, ''Peter Pan,'' is a mixed blessing. For the first hour or so, it is an up and down thing, excessive, clutterred, alternately exciting and listless.
By the time the film is half finished, however, it all comes together. The director, the cast and the set designers find the right key, and then the film is a pleasure.
Indeed, it is rather touching in the last hour, so you may be perfectly willing to wait for the film to take form.
The movie is a little dark, but then that shouldn't surprise those of us who saw ''Indiana Jones and the Temple of Doom.'' That was the Spielberg film in which someone extracted a heart from a man's chest.
Spielberg later said that piece of work was a mistake, but he may have made similar mistakes in the new film.
In ''Hook,'' Peter Pan is Peter Banning (played by Robin Williams), a 40-year-old attorney living in New York with his wife and two children. He is more lawyer than father, and when Captain Hook reappears and kidnaps Pan's children, Pan, with no memory of his early years, goes after them.
Hook, played by Dustin Hoffman, does a lot of talking about killing the children, and there is one sequence that may disturb young children.
Oh, well, today's children are tougher than those of other generations. They've lived with the ''Friday the 13th'' and ''Elm Street'' movies, so maybe they won't be bothered by what they see in this film.
The film is big. It was all done on sets, and you never saw any busier sets than these. They include a real pirate ship, built for $1.5 million, and most of the action takes place on a studio-built Neverland.
For a time, that's all the film has -- scenery. But after Peter Banning-Pan gathers those happy thoughts and learns to fly again, the film takes off with him. You can't help but soar with it.
Williams' Pan is reminded of who he was when he returns to England for a visit. When his children are stolen, he takes off for Neverland and the Island of Lost Boys with the help of Tinkerbell, who is played by Julia Roberts.
Roberts doesn't do anything special with the role, but then she is seen in miniature most of the time. Toward close, when she is big-girl size, she is far more appealing.
Hoffman is good, if not great, as Hook. Williams, however, is great. You might have thought, when you learned that Williams would be playing Pan, that he would never do, but he is a perfect choice for the role. He may have been rehearsing all his life for it.
They shaved his chest for the film. Peter Pan, no matter how old, can't be that hairy, so the mat went.
Maggie Smith is Granny Wendy, who looks much too old to be the Wendy Peter knew on his introduction to the Darling family, but because Smith is such an excellent actress, we believe her.
''Hook,'' whose very rich score, done by John Williams, is a great asset to this film, opens here today. It is a thing of dark and light passages, a movie that is cumbersome one minute, delightful the next. Fortunately, all the good things happen toward the end, so we come away feeling better, perhaps, than we should.
** The further adventures of Peter Pan after he has reached his yuppie years.
CAST: Robin Williams, Dustin Hoffman, Julia Roberts, Bob Hoskins, Maggie Smith, Caroline Goodall, Charlie Korsmo, Amber Scott, Phil Collins.
DIRECTOR: Steven Spielberg
RATING: PG (language, violence)
RUNNING TIME: 140 minutes