Our plumber friend who installed ours recommended putting it in the loft to free up space downstairs and said the only person who really needed to access it was him. It's worked great for 7yrs (touch wood)
For me it would depend on a few things - how good / easy is the access to the loft. Does the condensate pipe have a defrost wire thing (I'm sure it has a proper name but can't think what it is) - this is a problem with loft boilers as if they don't have one and it freezes in winter you can't reach the pipe so have to wait for it to thaw for your boiler to start working again.
My mum has the boiler in the loft in her house (loft room is bedroom so cannot comment on freezing etc) the only thing I notice is that it takes a long time to run the water through in kitchen before it gets warm enough to use. Might matter if you were on a water meter.
Mine was by my sink so near instant hot water so that's why I notice the difference.
Could see access being a problem if you wanted to advance the heating.
It would put me off. Having sensor and remote thermostats just means more parts to go wrong. I like to go for simplicity as less to go wrong and pay for in the long term. Frozen pipes and lack of access would put me off and i always try to make a loft room if possible so this would create an extra expense there also.
Thank you all, you've raised some points I hadn't thought of, and given me some questions to ask the vendors! It's a nice house but I did wonder if the boiler in the loft was a bit odd, it's obviously more common than I realised.
It's slightly inconvenient having to go up in the loft to check whether the pressure has dropped / system needs topping up etc. but that's not a frequent thing and it certainly didn't put us off buying the house.
I suppose it depends on the boiler. My cranky and unreliable 15 year old one has needed a lot of re setting and re pressuring other the 10 years we've lived in our house. This is not stuff we have needed an engineer for as we do it ourselves where possible/safe to do so. It would have been a massive pain if it was in the loft ( especially as I have a phobia about even looking up the hatch let alone climbing up there! )
Ours is in the loft and is a total pita- our plumber said it's dodgy having a boiler in the loft unless really accessible loft (ours has pull down ladder etc) saying that our boiler is old and frequently needs tampering with I'm going to pay for the new one to be moved downstairs