My thoughts:
I am British but now live in Bulgaria and am involved in property sales and renovations, but mainly in central Bulgaria and more rural properties - not along the coast.
However, I first became involved around 3 years ago when I reserved an apartment in Bansko. This was through a very well known and London based agency who advertise heavily in national papers. Well after numerous problems I eventually attempted to pull out. It took me around a year and a court case later to get my money back based on misrepresentation. The main problem was not the developer but the agent.
However through visiting Bulgaria numerous times I met many friends here and now work with a small agency in Central Bulgaria.
I agree with the comments that places like Sunny Beach are frankly bloody awful , although popular with certain Brits, Russians and Germans. There is massive over supply, some poor planning and bad builds but there are also some very good smaller developers with lovely places away from the main over built area.
I have many friends that now live just outside Varna, many Brits have retired here or moved here to start a new life.
Bansko has been totally spoilt. It will never recover, the National park areas have been built on, no money has been put into extra sewerage, water or electric systems and so there will be trouble later. Talks of extending the ski area and opening an airport have been around for years and never happened. The old town is lovely but apartments are very expensive at more than you can pick up an apartment in Sofia or other cities for.
Rural properties have increased steadily in value over the last few years but nothing stupid like on the coast. For better value move inland or at least away from the main tourist areas. Many Bulgarians buy these properties for second homes and investment also. It is not just a British market.
It is true there are Japanese living in Kazanlak - not far from me. I also have heard Ryan Air will begin flights into Plovdiv - centrally located and good for Pamporova in winter and areas like Yambol, Elhovo, Stara Zagora etc popular with Brits.
Rachel