By Kevin Connolly
BBC News, St Louis, Missouri
As a benchmark for sensible political judgment, the 100-day deadline in American politics is essentially useless.
Mr Obama has travelled throughout the US during his first 100 days
But since Franklin Roosevelt first used it to trumpet his own achievements in the depths of the Great Depression, we have been more or less stuck with it.
Even if a new president has been effective, we only know what sorts of legislation he thinks will work. We do not know how that legislation - like Mr Obama's $789bn stimulus programme - will work out in practice.
But it was interesting that Mr Obama took to the road to put his case to the American people and intriguing that he chose St Louis to do it.
Mr Obama lost here last year - only the second candidate in a century to win the White House without winning Missouri.
And his supporters here will remind you that he only lost by a whisker and they are in no doubt that he is determined to win it next time around.
So Missouri it was as the venue for the nationally-televised town hall meeting with which Mr Obama marked his 100th day in office.
The very fact that he chose to get out of Washington was a reminder of one of the keys to his style of governing.
He is the traveller-in-chief and persuader-in-chief as well as the Commander-in-Chief. I can remember key speeches and interviews in Florida, California and Colorado in this 100 days, and plenty of other visits besides.
It sometimes looks as though he is running the country as though he were still running for office - but the truth is that he is his own administration's best asset when it comes to explaining difficult policy choices.
Some sections of the American media have hit on the idea of marking the president in the manner of a school report card (a simple editorial device which is being promoted on one channel as though it were on a par with the invention of the wheel).
So it was appropriate that Mr Obama took the White House Press Corps and the heavy machinery of the travelling White House to the crowded gymnasium of the Fox High School in the St Louis suburb of Arnold.
The open town hall meeting format is an interesting one.
The events are open to anyone who cares to come - and as the president himself was quick to stress there is no pre-screening of the audience. So in theory at least he could find himself facing a barrage of hostile questions from angry Republicans.
In practice, demand for tickets at these events is so high and the business of queuing and clearing security so onerous that only the Democratic Party faithful seem to have the motivation to attend.
It is still true that any subject under the sun can be raised by the audience though.
Was it just me or did Mr Obama look a little surprised to find himself marking his 100th day in office with a public discussion of alternative medicine. (If you missed it, he offered a little cautious support for acupuncture but otherwise said he'd be "guided by science").
He is of course smart, articulate and supremely comfortable in this format.
He knows how to slow proceedings down and give himself time to think by praising the poise and articulacy of a girl from the fourth grade who asked a question about the environment or jokily thanking one of the Fox High School teachers for bothering to wear a tie.
And of course, he is a quick, and careful study - reportedly spending hours absorbing complex briefs on every policy area he has to deal with. He is not going to be caught out.
Mr Obama has a record of energy and purpose to set before the American people
On top of that, though - and I do not wish to spoil the 100 Days party here, or indeed spend the rest of my working day answering emails from outraged Obama supporters - he is long-winded. Sometimes very long-winded.
I timed one of his answers in St Louis (about the future of social security) at more than 10 minutes.
That means of course that it is not so much an answer as a discursive essay on the history of the issue and the vast range of policy choices which confront the administration.
It also means that at a town hall meeting he never has to answer more than a handful of questions.
His listeners get a real chance to hear him talking through an issue - it is rather like getting to eavesdrop on a meeting between the president and his advisers.
But they do not get to ask him many questions - and there are no follow-up questions at all, which makes the event relatively easy for him to control.
The contrast between the atmosphere during the president's opening speech and the Q&A session which followed was interesting.
The speech - billed as Mr Obama's own report back to the American people on his first 100 days - was punctuated with sure-fire applause lines about "the start of another long journey" and the fact that people know "we're working for you every single day".
The question session was notably quieter - not even the president's keenest supporters are going to be roused to applause by the sight of him musing on the philosophy that underpins the overseas aid budget, or indeed acupuncture.
Mr Obama has a record of energy and purpose to set before the American people and he can point to issues like the closure of Guantanamo Bay and the abandonment of harsh interrogation techniques to demonstrate that a line has been drawn under the days of George W Bush.
But it is on the economy that he will be judged and it will not be possible to judge the success of his key strategy there - borrowing big to spend America's way out of recession - for months, if not years.