The long-awaited green light for the £16bn Crossrail project has been hailed as the most significant decision for London in decades.
Some fear the costs of Crossrail will soar beyond £16 billion
Indeed the only question bothering many politicians, business leaders and commuters is why it has taken so long.
Prime Minister Gordon Brown says the scheme will create 30,000 more jobs, boost London's financial reputation, and regenerate its most deprived areas.
Crossrail supporters also predict soaring foreign investment and a spate of new property hot spots.
However not everyone relishes the prospect of the mammoth project.
Critics fear massive disruption and warn that spiralling costs will see funding slashed for other key transport and infrastructure initiatives.
And they point out that it will not even be ready for the 2012 Olympics, which is itself likely to test Londoners' pockets and patience.
Meanwhile, Tories question the timing of the move, noting the government has already promised Crossrail twice - on the eve of the 2001 and 2005 General Elections.
But Graham Capper of London First, which represents 300 leading businesses, sees no clouds on the Crossrail horizon.
"We have waited the best part of 20 years for this announcement. We are absolutely over the moon," he declares.
He adds: "It is great news for London, it is great news for London business, and it is great news for everybody that has to use public transport to commute in and out of London.
Crossrail is still a long way from its final destination
"As a world city London has started to get a reputation for being a nightmare to get in and out of.
"Around the world potential investors will see this announcement and realise that London and the UK are doing something about it."
Mr Capper says a substantial contribution from business is a "political necessity" for the government and London First believes a 3% supplement on business rates will be acceptable to most large firms.
But he adds: "We are expecting the government to be at the top of that envelope and maybe a little beyond. Of course turkeys don't vote for Christmas and some businesses may be unhappy."
He says project management will be crucial, but predicts: "This isn't going to be another Olympic-type project.
"The costs have been checked very, very thoroughly by highly credible people and we are confident that substantial contingency allowances are already being incorporated."
More capacity 'vital'
Michael Snyder, head of policy at the City of London Corporation, considers Crossrail is the most important new transport infrastructure in the UK for 30 years and is vital to the UK's future prosperity.
Transport delays are costing City businesses £1m every working day, he notes, but Crossrail will add £30bn to UK GDP over 60 years and contribute £12bn in tax revenue.
"It will help London cope with the forecast growth in population and jobs and will play a significant part in maintaining our position as the world's leading international financial and business centre."
But Respect MP George Galloway says there is serious concern about the scheme among his constituents in Bethnal Green and Bow.
"It is a hugely expensive prestige project and even if the government says it has got the funding sorted out, we do not trust that it is not going to squeeze other priority programmes for the East End," he adds.
Peter Lawrence, president of Railfuture which campaigns for better services, hails Crossrail as "a great step forward", but says limited track capacity means it may create "serious difficulties" for freight and other passenger services.
Lord Berkeley, chairman of the Rail Freight Group, goes a step further, observing: "As a passenger Crossrail sounds good and I shall probably use it.
"But the government is acting without building the necessary infrastructure for it and it is actually stealing capacity from rail freight operators.
"Government policy is to encourage and increase rail freight and they are doing the opposite here," he says.
Professor Stephen Glaister, professor of transport at London's Imperial College, says the development is "desperately important" given that an estimated 800,000 more people will live and work in London by 2025.
"All the transport systems, rail and bus, are pretty much full up in peak hours at the moment so we must find ways of expanding capacity to deal with that," he says.
Prof Glaister, who is a member of the Transport for London Board, stresses that Crossrail is only part of the solution and that substantial improvements in bus and other rail services will be needed alongside.
"The only thing I hesitate about is whether this announcement today is a final guarantee that we will get it built," he adds.
He says a third of Crossrail funding is supposed to come from the business levy - which will require separate primary legislation - and is bound to be controversial in Parliament and among businesses - especially middle-sized firms which could be hardest hit.