Dead-end, or the fast lane into the future? Opinion is divided over whether there will ever be a market for cars powered by hydrogen fuel cells.
Well, Toyota is about to go some way to giving an answer.
The Japanese giant announced at the Paris Motor Show that it will be putting fuel cell vehicles (FCVs) on sale in Europe next year.
Following announcements that Toyota's fuel cell sedan car will go on sale in Japan and California in 2015, the company has added the UK, Germany and Denmark to the list.
However, no one is expecting consumers to make a mad dash to their local car dealer for the four-door mid-size car.
Toyota's FCV ("fool cell vehicle" say critics) is powered by electricity created by the chemical reaction between hydrogen and oxygen, leaving water vapour as the only tailpipe emission.
Trouble is, the cars need re-filling with hydrogen, and infrastructure to do this is virtually non-existent. So why put them on sale?
Satoshi Ogiso, managing officer at Toyota Motor Corporation and a key executive behind the FCV, admits that next year's launches are baby steps. "But Toyota plays the long game," he told the BBC.
Mr Ogiso recalls the scepticism that greeted Toyota's development in the 1990s of the ground-breaking hybrid gasoline-electric Prius car.
Critics said hybrids would never catch on. But last month, Toyota notched up the seven-millionth sale of its hybrid models.
Mr Ogiso, who worked on the launch of the Prius, said: "The introduction of our FCV will be limited, step-by-step and gradual.
"We see very much a similar approach and expectations to when introducing hybrids back in 1997. It very much took time to embed the technology and develop the market."
FCVs are for the coming decades, he said. "But we are motivated to make a start and help evolve the market."
However, at least Prius drivers had a pre-existing infrastructure - petrol stations. The main challenge was convincing consumers that the technology worked.
Why would someone buy a FCV without refuelling stations? And why would anyone build refuelling stations without FCVs on the road.
Mr Ogiso accepted that the chicken/egg argument was a strong one. But he is not thinking in those terms. It's about multiple agencies marching in step, moving ahead as one.
There are 13 hydrogen refuelling stations in the UK, most of them linked to academic and industrial sites.
Several others are already under development, and Toyota says there should be about 15 publicly accessible stations by the end of 2015.
Denmark also has 15 stations planned, and Germany has confirmed 50 will be built by the end of 2015.
More than 100 are planned in Japan by the end of 2015, concentrated in major cities. In California, the state is spending $200m (£124m) building 100 stations, most in and around Los Angeles.
In the UK, the H2Mobility project, a government-industry initiative set up in 2012 to assess the needs of FCVs, has identified that an initial network of around 65 hydrogen refuelling stations would be sufficient for basic national reach in the early years.
Toyota itself estimates that to give FCVs a solid and lasting foothold in the UK, there would need to be at least one station within a 200km (120 mile) radius - mostly along motorways and main roads - with many more concentrated in urban centres.
Critics argue that the number of stations needed to satisfy consumer demand is being wildly underestimated.
No one wants to drive dozens of miles to re-fill, and, unlike electric vehicles, ever being able to top up at home is probably a fantasy.
Also, using hydrogen is a wasteful process, when compared to electricity, it is argued. What's more, while Toyota has dismissed mass use of electric vehicles because of the weight, size and cost of batteries, the company's critics say it ignores the complexity and expense of developing fuel cells.
Tesla's chief Elon Musk has said that of all the outcomes for FCVs, "success won't be one of them". Meanwhile, executives at Germany's VW have said that FCVs are unlikely to catch on outside Japan, where the government wants a "hydrogen society" with fuel cells powering offices and homes, as well as cars.
Toyota's FCV Sedan
- Front-wheel drive, four doors, four seats
- Fuel cell, two hydrogen tanks and battery fitted under the floor
- Can be refuelled in 3 minutes
- Max cruising range: 700km (430 miles)
- Max speed: 170km/h (105mph)
- 1997: Toyota unveils its first hydrogen concept car
Mr Ogiso accepts that there are challenges. "As Toyota and other manufacturers begin to bring hydrogen-powered vehicles to market, it is important that all interested parties continue to work together," he said.
"Through co-ordinated dialogue between industry partners and government bodies, we can ensure the benefits of the technology are more widely understood, appreciated and realised," he said.
Toyota will not be funding the infrastructure in the UK. It will be up to energy companies and local authorities to assess the investment and commercial return. So, until such an infrastructure is reached, Toyota's marketing of its FCV will be targeted and specific.
It means that early customers are likely to be what Toyota calls "our existing commercial partners" already using the company's low emission technologies. Mr Ogiso added: "We can also envisage interest from other 'green' company fleets and public authority fleets.
"We have set no specific target or expectation for the UK market. We understand and appreciate that the market development for FCVs, and related infrastructure, will be challenging at the outset.
"But we are strongly motivated to help highlight how FCVs can be a safe and relevant mobility solution moving us towards the development of the ultimate eco-vehicle in years to come."
And how much will they cost in the UK? Toyota isn't saying yet. In Japan, the fuel cell Sedan will cost about 7 million yen (£43,450). The price is being subsidised by the government.
But Toyota says the price does not translate to the UK, given taxes, import duties and other costs. Some industry experts have put the cost per vehicle at about £50,000.
Customers 'will decide'
The debate about which technologies will power cars of the future is becoming polarised between electric plug-ins and FCVs, which are also being developed by Hyundai and Honda.
But Mr Ogiso is no fan of this either/or argument. "We don't see this as a competition between the various technologies, and believe FCVs will coexist with hybrids, plug-ins and electric vehicles."
Indeed, much of Toyota's technological efforts still go into making standard gasoline cars more efficient.
"Customers will eventually decide which technology and vehicles meet the desired needs for mobility," he said.
As with car buying today, customers of the future "will make their choice depending on the size of the vehicle and range. What matters is that customers have choice, and Toyota will not move away from offering a portfolio of choices," he said.