Yes, I've had a Mitsubishi Ecodan for the last 8 years and have been happy with it. We've had no issue with it at all, but we do have it serviced (as per the RHI contract) each year. Friends have a Mitsubishi that's been working for 11 years now. You'll need to have a hot water tank installed if you haven't already got one. Your installer will talk to you about your household, how many baths you have each day and so on, and size it to suit.
Because it's a low temperature form of heating you may need to have some of your radiators replaced with something larger, or doubles where you currently have singles, to ensure that the rooms are warm enough.
You need to have a well-insulated house to enable the ASHP to work efficiently. You can't use one to heat a house that's like a sieve, heat wise. So having the cavity wall and loft insulation done, and replacing any single glazing with double glazing will all help. I think you need an EPC of at least D to qualify for the RHI, which pays back a chunk of money every year for 7 years. The amount depends on various factors. Our repayment almost completely paid for our system, so it's worked out eventually costing no more than replacing our old boiler would have cost.
We find our ASHP is relatively cheap to run, but then we also have PV which can help support it (and which makes us money to offset against the electricity bills). You have to adjust to using it because it's not quite the same as having a standard boiler. It takes longer for the house to heat up because the radiators don't get so hot. At the moment our heating comes on at 6am and turns off at 9am, then comes on again at 5pm for four hours. We can turn it on and off remotely via an app. We have the thermostat set at 20 degrees. We also have a wood burner for the worst of winter. When it's very cold the ASHP has to work harder to keep the house warm and so we put a few logs on to make the place extra cosy and take the strain off the ASHP.
If you're someone who wants your home perpetually at 23 degrees it'll obviously cost you more than running it on a lean basis. We try to be green, which is why we have an ASHP, and we're the kind of people who put on another jumper rather than turn the thermostat up.
We have friends who recently had the Rolls Royce of ASHPs, a Stiebel-Eltron, fitted and that's amazingly efficient and worth investigating if you are techie enough to understand the stats. Far more so than a Mitsi, although it costs more to start with. We'll have one when eventually the Mitsi dies.
You can find suppliers and installers on the MCS site:
mcscertified.com/find-an-installer/
All installers have to be a part of the MCS scheme, which is highly-regulated. Beware of anyone who isn't MCS certified because you won't be able to claim the RHI. I would always look for a local company that's been in business for a few years. Maintaining and servicing ASHPs is quite specialist work that your average gas/ oil heating engineer can't do. I know from experience that if you use one of the large apparently national companies, they'll just phone around looking for someone prepared to install your ASHP as cheaply as possible — with issues with back-up if something goes wrong. We know people in Dorset who had two engineers from Scotland drive down and stay in a B+B while they fitted their ASHP. When it wasn't working properly they struggled for months to find someone to sort the problem out.
Our ASHP was installed by a local company who will send someone out within 24-48 hours if there are problems. We haven't had any problems, but like all equipment they do occur.
You have to have your ASHP installed before next March to get the RHI (Renewable Heating Incentive) payments, so probably best to get onto the case now. There's be a major rush towards the end of the year, I expect.