There will be a number of different opinions, especially since he's been leading the Better Together campaign. But can we agree on a couple of characteristics? First, he isn't flashy. Secondly, like him or not, he says it as he sees it.
For example, in 2008, during the banking crisis, he told us: "The economic times we are facing … are arguably the worst we have had to face in 60 years." Mr Darling, who was Chancellor of the Exchequer at the time, thought it was his duty to be straight.
As a consequence, Gordon Brown's people "unleashed the forces of hell" against him. (This he also revealed.) But Mr Darling, as it turned out, was right.
His candour is one of the traits that should give Alex Salmond pause for thought about their upcoming television debate on independence.
There will be those in the Yes camp already predicting a victory for the First Minister. Mr Salmond is a natural and quick-witted debater, one of the best performers of this political era. I'd put him alongside George Galloway for fluency and, in a different way, Boris Johnson for his ability to take a complicated subject and present it in a way that wins over an audience.
Here are two examples of the First Minister's talents. On life in the UK post independence, he commented: "The honest folk of Scotland will be friends with the plain people of England."
It's folksy but exquisite, worthy of a final scene in The Hobbit.
On Westminster spending the taxes accrued from North Sea oil revenues, he said it was the "greatest act of international larceny since the Spanish stole the Inca gold".l
In his ability to produce for sound bites, he outshines Mr Darling. I imagine we can all agree on that, too. For that sort of contest, David Cameron would have been a closer rival. Then we could have looked forward to a gladiatorial clash. My money would have been on Mr Salmond even though Mr Cameron can also turn a phrase.
The Prime Minister is fast and sometimes funny but usually at his opponent's expense. Watch him swagger at Prime Minister's Questions and you'll see why some think of him as George MacDonald Fraser's anti-hero, Flashman. One December, he said of Ed Miliband: "He has completely united his party, every single one of them has asked Santa for the same thing, a new leader for Christmas."
This tendency to arrogance combined with his patrician voice, would be a gift to Mr Salmond. In the (unlikely) event that he scored more points than the First Minister landed on him, he would still lose the debate. The "honest folk" of Scotland would hate him for it.
Instead, assuming a date for the debate can be agreed, Mr Salmond will take his podium opposite the man with the white hair and black eyebrows. He will be debating, not with a Tory toff, but with a fellow Scot. Mr Darling is as much at home in the Western Isles as he is in Edinburgh or Glasgow.
Mr Salmond will be confronted by a down-to-earth character in place of Mr Cameron's tendency to glibness. For Mr Salmond, Mr Darling presents a tougher tactical challenge.
The former Labour cabinet minister has a track record in many of the areas where the Yes campaign is most vulnerable: currency, taxation, the effects of independence on the financial services sector, pensions and the benefits system.
Because of his low key gravity, the First Minister might be tempted to display his verbal acrobatics. But no- one reaches the political heights the former Chancellor has achieved without a strong intelligence and a tactical mind. Mr Salmond will need to beware the use of emotive word pictures and sweeping generalisations in the face of the former Chancellor's attention to detail.
Nor is the man two dimensional. There are descriptions that wouldn't immediately occur when Mr Darling's picture flashes up; words like passionate, tribal and witty. Yet all apply. In his quiet way, he is a passionate politician deeply committed to his party and to the Union. And, off camera, he is funny.
What will also matter in this debate is his lack of ego. I'm not saying he doesn't have one. All politicians do. But, unlike most of them (Mr Salmond included) it doesn't seem to precede him into a room. He gives every impression of being a modest man.
That characteristic will spike one of Salmond's readiest weapons. He won't be as able to deflect the debate into personal comments.
Who could forget Mr Salmond's put-down of Gordon Brown after he succeeded Tony Blair as Prime Minister? Mr Brown had been dithering about an autumn election and then decided against. Mr Salmond dubbed him "a big fearty from Fife". Then, devastatingly, he added: "Those whom the Gods seek to destroy they first render ridiculous, and this shambles leaves Gordon Brown looking totally ridiculous."
I'm not sure Mr Brown ever recovered. But if Mr Salmond tries those tactics against Mr Darling, he risks coming off second best. It won't be because Mr Darling will retort in kind, it will be because Mr Salmond will look as though he's trying to win a serious political argument by playing the man instead of debating the issues; as well as which the insults will bounce off. Remember, Mr Darling survived the forces of hell without breaking a sweat.
Mr Salmond will be forced to come back to the facts. The broad sweep will not do. Nor will clever slogans or sound-bites. If he resorts to quoting Burns, he will find Mr Darling waiting for the information that matters.
This is the point. Mr Darling cannot be dismissed easily. Mr Salmond, no doubt, would have accused Mr Cameron of presiding over the dismantling of the welfare state, of causing suffering to poor Scots. But Mr Darling, a Labour man, is a more difficult target. As Chancellor, he rescued the Scottish banks and, with only hours to spare, he safeguarded the deposits and savings of all of us.
This is not to eulogise him but to highlight the challenge facing the First Minister. As a voter in the referendum, I want a debate that deals with the complexities of independence. I want a discussion about issues and ideas. With Mr Darling, that's more likely what we'll get.
A television debate on an issue of such importance is not a contest of champions. It's to inform us properly on a choice of destiny. In 10 years, neither man will be in power (or it is highly unlikely). To that extent their personalities are irrelevant. We need to look and listen beyond them.
I spoke to a seasoned politician at the weekend. He told me he swithers between thinking the vote will be knife edge, to a Yes victory, to 60/40 for the Union. With so many people still undecided, who can call it?
That being the case, with September 18 approaching, it's essential that someone puts Mr Salmond and the arguments for independence under pressure. I think we have a decent chance of getting that now.
About time too.