"Bar Mitzvah Boy," just about the nicest musical in ages, opened here recently and may it run for 100 years.

You can sing the songs. And why not? Jule Styne ("Funny Girl," "Gentlemen Prefer Blondes," "Gypsy") wrote them.

You can believe in the characters invented by Jack Rosenthal; a cab driver's family from near Golders Green. You can smile at Rosenthal's gags, or at least a good many of them.

And Barry Angel, the Manchester grocer's boy who plays the title role with an honest heart warming awkwardness-well-he's a 16 year-old doll.

"Bar Mitzvah Boy" is as sweet as Hamantash and so what if it is sometimes as cleving as the honey in that Purim cake. For on occasion, Don Black's lyrics are as pungent as hot pastrami like when the children, complaining of parental oppression, sing. "I'm all tied up in thou shalt notes."

Sure, the second act drags a little-too few songs-and the dialogue can be heavy. But in this life you've got to take a little bad with the good.

Everybody in the cast dances and some of them-especially Zelah Clarke, the shiksa who wants Barry to know that if he's now a man. She's a woman-dance quite well. Everybody sings. So maybe Barry doesn't have to try hard for his cracked, adolescent tone, but Leonie Cosman, who plays his understanding older sister, can make even Styne's most banal tunes sound great. And Joyce Blair, Barry's frantic mother, can belt out a brassy ballad with the best of them.

You usually don't expect much acting from a musical. But Harry Towb, who sings like your ordinary barroom tenor, can almost make you cry when, tongue-tied, he tries to tell his bar mitzvah son how much he loves him.

London's West End is not the likeliest setting for what the critic from The Times loftily dismissed as an "'ethnic' comedy. . . a bland, ingratiating platitude machine." But remember that the most famous kosher restaurant here is in the East End and, frankly, it could not make the grade in Bensonhurst.

When "Bar Mitzvah Boy" reaches Broadway, where it is due to go next fall, it will find its natural home.

The show is based on a prize-winning play Rosenthal wrote for the BBC. Eliot Green (Barry Angel) is preparing for his bar mitzvah (for the London audience, can you imagine, they had to stick in a prologue explaining that the ceremony is a ritual passage to Jewish manhood) and hie is beset with doubts. His overwhelming mother (Joyce Blair) is only worried whether the dinner dance after the service will impress the 117 guests, and whether her new hairdo is a knockout.

But Eliot clings to childhood, hiding behind a Mickey Mouse mask in moments of stress. And who wants to be a man if manhood meansd being like his father, fretting over bills and clearly no god: or like his sister's spineless boyfriend, or his platitudinous granfather.

The show's best-staged scene is a synagogue, all blue and gold, with the men in the congregation wearing their white and blue shawls or tallis. At the climactic moment, just before he is to read the Torah. Eliot runs away.

A Talmudie judgement by the rabbi, Peter Wireman (who is unhappily confined to one sprightly tune. "If Only a Little Bit Sticks") brings about a happy, even schmaltsy ending.

At the Guardian. Michael Billington differed with his Times colleague, praised the story, praised the tunes and concluded with some justice: "It's not a great musical but it works and it actually seems to be about flesh-and-blood people."

Milton Shulman in The Evening Standard tried hard not to like the show, complaining:

"Trying to stretch this anecdote into a full-blown musical has left it with soggy padding in the middle and anrch sentimentality in its final passages,"

But then Shulman concluded, "It would be churlish of me not to recommend a musical that made me laugh so much."

John Barber in The Telegraph said he was left with the "inescapable sense of an undernourished show on an oversized staged."

But B. A. Young of the Financial Times, the most respected daily critic in town, called his evening "uniquely enjoyable," and delighted in everything he saw and heard.

No doubt the Ocedipal finale-mother and son dance together at the bar mitzvah reception to one of Styne's stickier songs-is a letdown. The best all-cast number, the bright and bouncy "The Bar Mitzvah of Eliot Green," turns up in Act One, Scene Two. It is also true that the bitter-sweet quality of adult compromise which distinguished and gave bite to Rosenthal's TV drama is buried under layer cake here.

But never mind. You still come in humming, "I've Just Begun" and "This Time Tomorrow." And for once, you really can bring the whole family.