President Zelensky insists the system is vital to his country's efforts, writing on Facebook that "Himars and other precision weapons are turning the course of war in our favor".
By mid-August, Ukraine's forces had destroyed more than 100 high-value Russian targets with Himars, according to the US army.
"Himars helps cut supply lines, and that makes it a very important weapon," says Prof O'Brien.
"It has given the Ukrainian forces a new strategy - of degrading the Russian supplies until they cannot hold, of depleting Russian firepower, and of wearing its army down by attrition.
However, Dr Marina Miron, warfare expert at Kings College London, believes its impact may have been exaggerated.
"Himars is very accurate when it is used against fixed targets, when you have the exact co-ordinates," she says.
"But it is not so effective against moving targets such as troops, so it cannot push back an advance.
"I would say that Himars caught the Russians by surprise, but it doesn't change the balance of power."
Image source, Getty ImagesUkrainian defence secretary Oleksii Reznikov says Ukraine needs 50 more Himars systems to stop Russian forces advancing, and 100 more to launch a successful counter-offensive.
However, according to Prof O'Brien, the supply of the missiles used by the system is more important.
"Having 16 or so Himars launchers will be fine," he says, "as long as there is a tonne of ammunition to go with them. But the missile manufacturers are under a lot of pressure to keep up supplies."
How is Russia fighting back?
"Russia is now focusing on destroying Himars launchers and missiles while they are being delivered," says Dr Miron.
Its forces have repeatedly claimed to have destroyed a number of launchers and as many as 100 missiles, but have provided no proof.
"The Russians don't seem to be very good at combatting Himars by destroying them," says Prof O'Brien.
"Because they cannot fly aircraft safely over battle areas, they haven't been able to spot them from the air.
"And it is hard to hit back at a Himars once it has fired on you, because it can move almost immediately."
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How has Ukraine used Himars?
Himars began being used in Ukraine at the end of June or the start of July, according to Phillips O'Brien, professor of strategic studies at St Andrews University.
He says the system was first deployed against fixed targets and command centres, rather than forces on the move:
"It was used up and down the front in eastern Ukraine, striking at ammunition depots some 30 miles behind the Russian lines."
However, he says, Ukraine may now be starting to use Himars against concentrations of Russian troops in barracks and command posts.
Ukraine claims a missile attack in Lysychansk, Luhansk killed up to 100 troops, and that a strike on the headquarters of the Wagner mercenary group in Popasna in Luhansk may have killed 100 of its members.
Both strikes are likely to have involved Himars, according to Prof O'Brien.
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