Daisy, I lived in Bulgaria for a year around 2002. I lived in Sofia, the capital, and was perfectly happy there. Back then it was a lot less developed than the UK, for a big city everything was on a much smaller scale, felt a lot more provincial than UK cities at the time, but I imagine it's changed a lot since then. Bulgarian colleagues and my students were, on the whole, lovely. A few of my British colleagues had local partners and were settled there long term.
It's a beautiful country, lots of historical old towns, cobbled streets, etc. Mountains (skiing in winter, walking in the summer), beaches. Lovely warm summers and cold, snowy winters. English wasn't massively widely spoken when I was there, Russian was the second language, again that might have changed. Bulgarian language is Cyrillic script, looks impenetrable at first, but actually once you've learnt the Cyrillic alphabet, it's not too difficult at all.
It's poorer than the UK, some people have very little, but it's a developed country so it's poor in relation to the UK, but not, say, some South Asian countries. Not too different to the UK in terms of culture, how people socialise, what people spend their money and free time on.
Anyway, that's just my view, and a very old and possibly outdated one at that! It's also joined the EU since I was there, so I'm not sure what impact that might have had on the country. But I really enjoyed my year there and wouldn't be horrified at all if I had to go live there again.