Yes, I DIY fitted one in my living room 5 or 6 years ago, which also serves as my office during the day if I'm working at home (which I usually do, unless I have a customer or meeting to attend). It allows me to heat just the living room, and it is very efficient and cheap to run. Also allows me to cool it in the summer, but to be honest, there are usually just a few days of cooling required, verses several weeks of heating through the winter.
For heating, it works well except for an outside temperature range between 0C and 5C, where the outside evaporator will ice up with condensation, requiring it to run through defrost cycles, at which point it becomes inefficient. Once the outside temp drops below 0C, it works fine again (although I didn't realise this for first couple of years).
I have it plugged in to a power meter, and ISTR it used about £15 of electricity all through one winter.
So, very pleased with it. Mine came from B&Q when they had £200 off. B&Q only did them for about 2 years, but were it not for the £200 off, specialist suppliers would have been cheaper, plus they go to the effort to do the paperwork for 5% VAT on air-sourced heat pumps, which B&Q didn't.
The other thing I looked at is these units were much cheaper if you can drive down to some of the warmer parts of Europe and buy them there, where they aren't seen as luxury items. Don't know if that's still true today.
The DIY fitting isn't particularly easy (that's why B&Q stopped doing them). You can get units professionally fitted too (a colleague had that done), but the installation cost is several times the equipment costs, so that works out much more expensive.
There are some rules on what you can and can't do with refrigerant if you don't have the relevant C&G, but the rules do allow you to fit preloaded aircon units with self-sealing connectors (although bizzarely they don't allow you to check for leaks).