Shake Shack, 24 Market Building, Covent Garden Piazza, London WC2 Five Guys, 1-3 Long Acre, Covent Garden, London WC2

With its subfusc grey frontage and terraced al fresco seating, Shake Shack blends right in to its environment in Covent Garden and has marketed itself as the foodie's choice

People think this job is easy, but it's hard, I tell you, damned hard. My mission this week: to spend an entire day queuing for, and then eating, burgers. Like Morgan Spurlock in a hurry, I will be attempting my own Vine-length version of Super Size Me.

The two burger hotspots I'll be queuing outside are both Stateside legends – upscale New Yorker Shake Shack, and homely Virginian Five Guys. Over-hyped, over-seasoned and over here, these US invaders opened within a day of each other in London's Covent Garden, to a supersized reaction normally reserved for liberating armies. There was a two-hour queue outside Five Guys on opening night; a fast-feeding frenzy not seen in Covent Garden since Balthazar's advance-booking lines got swamped by angry restaurant critics.

I hit Shake Shack first, joining the lunch queue at the prized time of 11.40 on a Monday morning. The site, in Covent Garden's glass-roofed market building, is primer than the grass-fed Aberdeen Angus used for Shake Shack's burgers. Round here, the trade isn't just passing, it's staggering around stunned, as disoriented tourists escape the attentions of buskers and unsettling human statues.

With its subfusc grey frontage and terraced al fresco seating, Shake Shack blends right in, apart from the queue of sweaty early-adopters. But the line moves quickly, wrangled by a smiley charmbot, and I'm at the counter within 10 minutes. Waiting for my order on the pleasant wooden terrace, listening to a street entertainer assaulting an aria from Madam Butterfly, it's almost like being in a proper café, rather than a fast-food place. Until I get my food.

The overall effect of the Shake Shack cheeseburger – of salt and grease and yielding mushiness – sets all the pleasure receptors buzzing. But it's Pavlovian; the flavour never arrives. For all the hype about Shake Shack's suppliers – the Scottish beef, the rare breed pork, the collaborations with local food heroes like St John and Paul A Young – this is a big, greasy bag of disappointment. The meat has the flabby, pre-chewed characterlessness of a mass-produced patty. The bun is sweetish, softish and a bit supermarketish. A couple of slices of tasteless tomato and a lettuce leaf add nothing. Only the crinkle-cut fries – ribbed for extra pleasure – are any good.

I'm already feeling a bit sick. But onwards, to Five Guys. Wow. Now HERE is a place you could never get tired of hating. Straddling a corner site at the top of Long Acre, it is hard and shiny and garish and loud, a red-and-white chequered circle of hell. That you have to queue to get into. And then queue again to collect your food.

Shunning the way of Morgan Spurlock, I get them to undersize me, by ordering the 'little' cheeseburger at £5.50, plus Five Guys fries. Again, the fries are good, in as much as they taste of actual food. But the burger is just warm pap; striped from the grill, it might as well have been steamed. What flavour there is comes from the toppings – 15 are on offer and you can order any or all of them for free. The menu also warns that all burgers are available bunless, an offer about as appealing as John McCririck being available shirtless.

It's not exactly a news story; restaurant critic hates fast-food restaurant. So let's not dwell. Let's pass over the ersatz folksiness of the big sacks of potatoes and boxes of peanut oil, which make a claim for Five Guys, with its 1,000-plus branches, as a down-home, rural, meat and potatoes (and peanuts) kind of place. Let's try to block out the memory of that soda fountain, dispensing unlimited amounts of 100 different flavours of soft drink, all of them revolting. And let's try to forget the betrayed look on the faces of the red-capped staff, who've signed up believing they'll be part of an exciting new food concept, and have somehow ended up working in McDonalds.

Instead, let's gaze in wonder at that queue. What are they thinking of?

A case study in successful brand building, these new arrivals will probably do brilliantly. More branches are planned for both. Shake Shack in particular has been clever in marketing itself as the foodie's choice – rather like Krispy Kreme donuts did when they first launched in the UK and were only available in Harrods, making them cult objects of desire.

But thanks to the new wave of home-grown burger joints that have opened in recent years, including superior chains like Byron, we've got used to our burgers being a bit better than this – nubbly and charred and actually tasting of beef. For all their branding, Five Guys and Shake Shack are both offering fast food. And here's my bulletin from the frontline. Fast-food burgers don't taste very nice. And they don't make you feel very good after you've eaten them. No shit, Spurlock.

Shake Shack, 24 Market Building, Covent Garden Piazza, London WC2 (0207-2400054. Around £10 a head for burger and fries

Food: **
Ambience: **
Service: ****

Five Guys, 1-3 Long Acre, Covent Garden, London WC2 (0207-2402057). Around £10 a head for burger and fries

Food: *
Ambience: no stars
Service: ****