Few can doubt that it was Brown's single-mindedness that brought him to the second most powerful job in government.
As finance minister, know in the UK as Chancellor of the Exchequer, he has been praised for restoring economic stability and funding public services.
But some on the right worry that as prime minister will might ditch the reforms of New Labour in favour of policies in line with Scottish socialism.
They say Brown, unconstrained by Blair's drive to make Labour acceptable to the aspirational classes, will find ways of taxing the rich and not-so-rich so that he can continue his drive to redistribute income.
On the other hand, Brown upset many by persisting with policies that hit pensioners and motorists before a course correction in 2000.
|As chancellor of the exchequer, Brown has|
generously funded public services [AP]
Brown, despite coming from a more traditional Labour background than Blair, was behind some of the policies most hated by the left, particularly cutting benefits to the disabled.
As shadow chancellor, Brown had worked hard to present himself as a fiscally competent chancellor-in-waiting, to reassure business and the middle class that Labour could be trusted to run the economy without fuelling inflation, increasing unemployment, or overspending - legacies of the 1970s.
He committed Labour to following the Conservatives' spending plans for the first two years after taking power.
Still others say that his tendency to keep himself within a circle of supportive colleagues will shut him off from the advice and criticism that a leader needs.
Others, mainly in the US, say that Brown's idea of "the special relationship" is that the US and Britain should join together to lead the world in fighting poverty, to the neglect of the fight against "terrorism", and devote their resources to lifting Africa out of poverty, rather than reinforcing his country's military.
Ties with Blair
It has long been rumoured that a deal was struck between Blair and Brown, in which Blair promised to give him control of economic policy in return for Brown not standing against him in the leadership election.
For a long time he feared that Blair was not ready to step down in his favour while division between the two men over Britain's entry into the single European currency was an open secret.
Whatever the truth, the relationship has been central to the fortunes of New Labour. They have mostly remained united in public, despite reported, serious, private rifts.
However, they could not stop New Labour from being portrayed as two factions - the Blairites and the Brownites.
Those backing Brown tended to be from the left, or the Old Labour wing of the party.
Brown has suffered a number of setbacks and his dour image has done little for his personal ratings.
He is often seen as a politician fixated on his job at the expense of a normal life.
That impression only began to change when he married Sarah Macaulay, his long-time girlfriend, and it eventually disappeared with the birth of his daughter.
The subsequent death of the child also brought sympathy from the public.
The fast stream
The son of a Church of Scotland minister, Brown attended Kirkcaldy High School, where he was placed in an academic fast stream.
As a student, he suffered a detached retina, possibly in an accident playing rugby. He was left blind in his left eye. A later operation for a detached retina in his right eye saved him from a total loss of sight.
Brown graduated from the University of Edinburgh with first class honours, and stayed on to complete a doctorate.
He lectured at Edinburgh and then at Glasgow before working as a TV journalist. In the 1979 general election, he stood for the Edinburgh South constituency, but lost.
He was elected to parliament as a Labour MP for Dunfermline East in 1983, becoming opposition spokesman on trade and industry in 1985.
Brown was the shadow chief secretary to the treasury from 1987 to 1989 and then shadow secretary of state for trade and industry, before becoming shadow chancellor in 1992.