In reply to cabbagepuller:
As above, really. I've been using them for a couple of years, and really like them for 'fun' runs because they're so light and unobtrusive. But for longer distances, or trying to go as fast as possible, I'd prefer to wear a more fulsome shoe.
Generally, I'd say they're most useful as an supplement to running training by more conventional methods, rather than an outright replacement. Not really better or worse, particularly, just 'different'.
They're also nice as a shoe for general wear, mind. You don't have to run in them if you're not keen on the feeling.
And remember, being all righteous and superior comes naturally when you've just shelled out £££s on footwear that doesn't really look like it should cost as much as it does and everyone thinks is silly. You just have to suppress the worst excesses and take quiet comfort in your rippling calf muscles.