With all the gasping and panting I do while running nowadays, I have to drop down to a slow walk while drinking. In the Derwentwater 10 last weekend I definitely lost places by stopping to drink at 5 miles, and got passed and beaten by someone who'd never beaten me before.
Having said that, in the London marathon, where bottles of water were supplied, as opposed to cups, I could manage to drink while running and never lost places.
I think what would be a more reasonable comparison (and perhaps what Steve was implying) is whether it would be beneficial to stop and take a drink or to continue straight through a drinks station without taking a drink at all...
In which case i'd say it very much depended on your time running and also weather conditions. As we all know dehydration is baaaaaaaaaaaaad, and even slight dehydration can have a large impact on performance. For me though I certainly won't bother drinking anything during a 10k (or less) and only drink perhaps at one drinks station on a HM (unless it's hotter). In either case I never stop to drink, I do my best to drink as much as possible form the plastic cups. As runningfox says, it's a darn sight easier when bottles are on offer!
It may be biased to say it, but I think the lesser fit people may benefit more from stopping to drink, not only from taking on the fluids but the chance for a quick breather too, as you get higher and higher up the field, it'll become harder and harder to pull back those elusive seconds lost by stopping for water.
it depends on what you're drinking out of. cups cause more problems than it's worth. I cough and splutter all over the place when ever I do this. I reckon it cost me a good 1/2 hour or so when I ran my last half. I'm sure I would have won the race instead of coming in in the hundreds.
I stopped for a drink and to sort myself out at Chichester, I wasn't running well at all and felt like pulling up. But after getting myself together I decided to catch the bloke up who I had been behind before with the 'Are we nearly there yet' T-shirt on. I did this, and felt chuffed with myself, but after time went on, I looked down and realised he had different trainers on, in fact totally different bloke, but same T-shirt! oh well I tried. Never saw the other chap again, and of course if I hadn't stopped I might have overtaken him! won't ever know.