the fact that the Americans run the Olympics at a profit doesn't (regrettably) mean the British will do the same. I suspect we'll have a few surplus stadiums sitting round at the end. Even Australians, who are sports-mad, haven't found a use for all the infrastructure they built in Sydney, I believe. But it's high time Britain did something high-profile like this; it will draw world attention to the country.... er, that's if it's not pouring with rain for the whole fortnight. And domestically it should encourage younger people to play sport, which may in the longer run improve their health and take away a bit of the burden on the NHS.
And, generally, it's likely to cheer people up a bit.