The problem is with these things is that they have to some degree been mis sold.
Yes they can work well. But sometimes they don't. As many people have pointed out, insulation is an issue. Radiator size is too. There is also the problem of maintenance, whether there are enough engineers around to maintain (some people in some areas have a lot of issues at the moment, but hopefully that will decrease with time). Then you have the actual maintenance costs, which are probably going to be more than a boiler because they have more moving parts under more stress.
It's hard to save money on the running costs, because getting a performance greater than 3 (3x the amount of heat out for the amount of electricity put it) means you have to have it set up well. Since gas is about 3x less than the price of electricity it is hard to save against mains gas.
People talk about the fact that they run well when insulation is installed, but you would also reduce the amount of gas you would use considerably if you installed equivalent insulation with a gas boiler.
They make some sense if you don't have access to mains gas. But still, you need to be careful as to whether the insulation you have in your house and radiator set is actually up to the job.
There is also the possibility that in the future the government will start to penalise people running gas boilers in order to force people over to heat pumps. There was talk of a £100 a year tax on boilers to try to get people to do this (in fact, this would be nowhere near enough to force me on to a heat pump). The other way they may do it is to change the gas : electric price ratio by increasing the price of gas, at the moment it is about 3.5, but they could gradually increase the gas price to force people onto electric. No one knows if this will happen.
If I had the money and had an existing working boiler, to me it makes more sense to spend money on insulation to make your house "heat pump ready" rather than go for the full conversion all at once. You will gain on your gas bill, and be in a position to swap out for the heat pump when it becomes advantageous to do so.