It has a worldwide reputation for high-class (if high-priced) Japanese cuisine. But, with the opening of its new London branch, has Nobu lost its mojo?

It even happens at the new Nobu Berkeley, with one profound difference. As you are taken upstairs to the dining- room, your escort is the one to yell "irasshaimase", prompting the staff around the room to call back. It's done, in other words, for appearances.

But then, Nobu Berkeley is all about appearances, isn't it? The David Collins designer makeover is quite delicious, transforming what was a nightclub and a Chinese restaurant into two levels of Parisian chic. On the ground floor is a holding pen - sorry, bar - filled with attractive girls in high heels and little black dresses. They would be the waitresses, then. There are poetic metallic light installations and gleaming columns that reference nature, and open island booths scattered like picnic tables in the park. A no-bookings policy means the bar soon becomes a noisy scrum. They could make a fortune here every night by auctioning off the restaurant tables - but perhaps not as much as they make by selling watery Matsuhisa Martinis at £10.50 a pop.

Up the rather lovely stairs, the dining-room is a vast, low-ceilinged space flanked by a spot-lit sushi bar. It's a glam little crowd, the girls in sequined shrugs and strappy sandals, and the men in blatantly expensive shirts over blue jeans. You can tell the Nobu habitués by the way they barely glance at the menu, knowing already they will have the new-style sashimi, lobster, wagyu steak, Comte de Taittinger and chilled sake.

I, on the other hand, get a severe case of menu anxiety, reading every word of the 17 different categories from toban yaki (ceramic hotplate) dishes to anticucho Peruvian-type skewers, including a new-to-Nobu list of dishes from the wood-fired oven. There is also a range of cook-at-the-table dishes, but you have to be at the special hibachi table for that, which I am not.

More wagyu beef goes by my table, at £26 per 75g, and more lobster wasabi at £29.50 a pop, but I can't afford such status dining. Instead I'm trying to find out what's good, aiming for a balance of new and old, raw and cooked, spicy and non, affordable and barely so.

I pass on the champagne and move to a crisp and grassy New Zealand Eradus 2004 Sauvignon Blanc (£31) from the somewhat uncohesive wine list. I was right to be worried about my food order, however, for what follows is a disjointed, mixed bag of overly twiddly dishes that promise more than they deliver.

From "Salads", Japanese turnip salad with whitefish (£7) comes as a clockface of five slices of turnip (kabu), each topped with a furl of raw whitefish gathered around a central outcrop of oakleaf, itself topped with wiggles of shaved bonito flakes. It tastes as you would imagine raw turnip and fish would taste.

From "Hot Dishes', jumbo grilled asparagus (£8.50) is a hash mark of cut spears, oddly accessorised with dried miso spacedust and a spoonful of vague-tasting "vegetarian caviar" prepared from the seeds of the broom-ash plant.

Next is a party in a bowl; a deep dish of rock shrimp tempura (£8.75) golden deep-fried nuggets nestling on green leaves. Three dipping sauces - a citrussy ponzu, tangy jalapeno and a spicy cream at least add a bit of zing.

From the wood-fired oven comes a big and un-Japanese-looking roasted and boned "crispy skin poussin" (£16) with a lemony, garlicky skin and a light and lovely stuffing of finely shredded cabbage. It is like a Sunday roast in miniature, complete with vegetables - okra, chicory, onion, tomato and potato. In spite of feeling more steamy than crispy, it's an amiable, unchallenging dish - but more Strada than Nobu, I would have thought.

We finish on a platter of good, bright chu-toro sushi (£4.75 a piece), fingers of bright pink sweet shrimp sushi (£3) and scallop and smelt roe maki rolls (£6.50). Surprisingly, the rolls appear inexpertly made, and quickly fall apart.

A chocolate bento box (£9) turns out to be a refined chocolate fondant with a scoop of green-tea ice cream in a bento box, but last week's chocolate fondant at The Princess in Shoreditch was much more delicious, for half the price.

Nobu is now a Malibu-to-Milan restaurant chain, a magnet for the rich and thin that is capable of producing truly great Japanese food. Frustratingly, that doesn't appear to be what drives it any more. The new-to-Nobu dishes seem done for effect and don't deliver the required Nobuliciousness, the prices are shameless, and the attitude seems more commercial and less Japanese, with some of the most aggressive glass-filling and bottle-emptying I have seen. Such faults are fixable, of course, but only if the management agree that they are indeed faults, and not a corporate business plan.

13 Nobu Berkeley 15 Berkeley Street, London W1, tel: 020 7290 9222. Dinner served Mon-Sat. Around £150 for two, including drinks and 15 per cent service

Scores 1-9 stay home and cook 10-11 needs help 12 ok 13 pleasant enough 14 good 15 very good 16 capable of greatness 17 special, can't wait to go back 18 highly honourable 19 unique and memorable 20 as good as it gets

Second helpings: More modern Japanese

Geisha 3 The Broadway, Lace Market, Nottingham, tel: 0115 959 8344 This glossy, glam club/lounge/restaurant was recently named Best Newcomer at the 2005 Nottingham Restaurant Awards. Kick off with a pink rhubarb-sake cocktail before moving into the moody, low-slung dining room. The menu is very Nobu-like - new-style sashimi, terrific lobster tempura, black cod with miso - as well as some pan-Asian offerings.

Sukoshi Frogmore Street (off Park Street), Bristol, tel: 0117 927 6003 Sukoshi is a party-time champagne and sushi bar, Japanese style. Cool and minimalist, with a wall-length water feature, it attracts a bright, young Bristol crowd who love matching their suski (prawn tempura rolled with crab, sushi rice and mayo) with their Sukoshi champagne cocktails, and their beef teriyaki with their chilled Kubota sake.

Roka 37 Charlotte Street, London W1, tel: 020 7580 6464 "Baby Zuma" is still pulling in the crowds with its contagious mix of shochu cocktails, robatayaki grill cooking and wacky wood-and-paper décor that mixes contrasting textures and tensions as well as the menu. Mastermind Rainer Becker knows how to make Japanese food contemporary without sacrificing integrity. Don't miss the rice and crab hot pot, grilled miso-braised daikon, and spicy lamb cutlets.