Airport hotels have always been depressing places, full of people reluctantly trapped on their way to somewhere they'd rather be. The bars are kept in perpetual twilight so jet-lagged businessmen can drink at any time of the day or night; the corridors are lined with trays of half-eaten club sandwiches waiting to be cleared; the bored staff know repeat business is governed by airline schedules not service levels. Then there are the prostitutes, the car park views, the scratchy polyester bedcovers...

But forget all that because the airport hotel has just been reinvented. In the past month, two new hotels have opened at British airports, both making extravagant claims about being luxurious, stylish - 'sexy' even.

More than that, both aspire to being destinations in their own right, hotels so wonderful that people will come and stay even if they are not flying off anywhere in the morning. Can such a dramatic metamorphosis be possible?

The big boy is the new, five-star, £180m Sofitel, linked by a bridge from the arrivals area of Heathrow's Terminal 5, and the only onsite hotel. On paper, it certainly sounds in a league above the traditional airport offering. The interiors were created by Khuan Chew, who designed the Burj al Arab in Dubai. The building was the work of Stephen Williams, the architect behind the Conrad in Chelsea Harbour. There's a restaurant from Albert Roux, a tea salon with 40 speciality brews, and a suite with Swarovski crystal taps that costs £3,525 a night. 'It's not an airport hotel; it's a five-star luxury hotel which just happens to be at an airport,' says Surinder Arora, the owner.

Meanwhile, overlooking the runway at Farnborough Airport in Hampshire is another new property, with scarcely less impressive credentials. The Aviator is the latest work of Ken McCulloch, founder of Glasgow's One Devonshire Gardens, Monaco's Columbus Hotel, plus the Malmaison chain. Farnborough is used for private planes, and the hotel hopes to convince visiting celebrities and business bigwigs that they don't need to travel into the West End to find suitably swanky digs.

Arriving at the Sofitel I'm slightly reminded of the BBC's excellent current series, House of Saddam. There are acres of highly polished marble, crystal chandeliers, scores of staff in shiny uniforms, glass lifts, numerous water features and no fewer than 32 full-sized indoor trees. The scale of the place is undeniably impressive - there's a conference hall with space for 1,700 people, 45 meeting rooms, plus an onsite theatre. It could all be a bit bewildering, but the simple layout and five vast atriums lend a feeling of calm. Unusually for an airport hotel, there's also a spa, from Espa no less, and the massage I have there is as good as any I've had. You can even stop off before your flight and get a Botox injection.

But, like all big hotels, it struggles a bit with the details. The rooms are smart, if a little bland, with vast beds and gorgeous linen sheets. Unfortunately there's a ridiculous switch system that controls all the lights from a central panel, which means that when you get up to use the bathroom in the night, you end up turning on every light in the bedroom. There's a TV in the bathroom too, but I couldn't work out how to turn it on. The windows can't be opened, so despite the great bed and sheets you'll be gasping on dry air-conditioned air all night (exactly what you don't want ahead of a long flight). There's a minibar but it's empty (the idea is that you call down and ask for it to be filled - but isn't that called room service?).

Shortly before I leave, I ask John Donaldson, the executive director of the Sofitel, what he makes of the Aviator. He hasn't heard of it, which is a shame, because in a five-minute spin around the comparatively modest Farnborough property, he would see more dramatic design, clever thinking and innovation than in the whole of the giant Sofitel.

Looked down on from the sky, the Aviator is the shape of a huge propeller, a nod to Farnborough's illustrious history as Britain's first airfield (it opened in 1905). Inside, at the centre of the building, is a round atrium, theatrically lit, representing the propeller shaft. This may all sound rather daft until you realise that this propeller conceit means that all of the building's rooms have interesting, fluid shapes instead of being square boxes. Though this is a brand new glass and steel building, it instantly has character.

The interior design, by McCulloch's wife Amanda Rosa, makes the place feel more special still. The bars and bedrooms have floor-to-ceiling windows behind dark wood Venetian blinds. The furniture feels individual, not ordered by committee - my room has two striking white patent leather chairs, plus two lovely standard lamps.

As anyone who has stayed at a Malmaison knows, dark, moody lighting is a McCulloch trademark, and it's used here to great effect. In the 'Skybar' separate seating areas in pools of soft light are surrounded by darkness, so what is actually a large, open-plan room feels as if it's full of intimate, secluded corners. Spotlights pick out huge bowls of bright pink hydrangeas.

The rooms are delightful and large, and half of them have great views over the runway. Largest of the three room sizes is the Skystudio, which at 65 square metres trounces the 32 square metres offered by the equivalent room type, the Luxury, at the Sofitel.

So while the Sofitel feels like somewhere to do business, this feels like somewhere to come and let your hair down at the weekend. And the room rates are so good, for now at least, that you'd be tempted to do just that - Skystudios are currently £155 a night at weekends, including free wi-fi, free films on the vast TV, and a free mini-bar (which does come stocked). Plus, joy of joys, the windows actually open.

But this isn't an entirely one-sided encounter. The Aviator makes great claims for its restaurant, which it hopes will be good enough to draw in local clientele for special occasions. But at the moment it's far from memorable. The sea bass, at £18.50, was one meagre fillet, the aged rib of beef, £16.50, was too chewy. The coffee took 20 minutes to arrive, and every 15 minutes we were interrupted by the maitre d' barking 'Everything all right here?'. Eggs Benedict in the morning (at £15!) came drowned under a slick of oily sauce.

And though in places the Sofitel might feel blandly international, once you step into its Brasserie Roux restaurant, you are very definitely in France. 'To go with the pike, which is, like me, from Lyon, you must have something from the terroir, a Crozes-Hermitage blanc,' explains Thierry, the ex-Gavroche maitre d'. 'It is just what the doctor ask.'

While the Aviator menu plodded through the usual favourites, from smoked salmon to roast chicken, here the menu has unexpected delights like pike mousse in a lobster sauce, duck foie gras and figs, and monkfish cassoulet (which is delicious). In fact the food, the charming French waiters, and the wonderful wine are so good, we forget we're in a newly built hangar just beside the Heathrow runway, and it starts to feel as though we are actually on holiday. Perhaps airport hotels don't have to be so depressing after all.

· Sofitel T5: 020 8757 7777; www.sofitel.com (doubles from £152). Aviator: 0870 220 8000; www.aviatorfarnborough.co.uk (doubles from £95).