I entered Turkey briefly from Greece and then cycled up through Bulgaria to Sofia, then into Serbia and onto Romania, Hungary etc.
I found Bulgaria to be a very mixed country - very evident, desperate poverty in many of the smaller towns, beautiful countryside and a somewhat threatening feel to some areas, particularly in the big towns - I was told by some locals in Sofia that this was the strong mafia influence. Not that you need to be particularly wary - there's no real threat from the mafia to someone passing through on a bike, but it's useful to be aware of it. It does make you see the place in a different light, knowing that it's so strong an influence.
I stayed in hotels because they were cheap. I did have camping stuff with me but I don't remember ever seeing an 'official' campsite. Wild camping would be possible - you'd just have to pick your place carefully, as in other countries, although I would perhaps be a little more wary of being disturbed by people wanting to nick my stuff than in, say, France, where the main concern would be a farmer wanting me off his land. The towns are very interesting though and they all have a different atmosphere that you notice as you pass through the country, so it's worth spending some time in them. That said, some of the hotels were disgusting.
Serbia was similar, but I only touched the top right corner for three days or so.
I met a Swiss cyclist in Bulgaria who had come from Albania. He said the accommodation was awful but that it was a beautiful country in parts. I think he had come from the coast.
Highlight of Bulgaria for me was the ride from Sofia north to Vratsa - an amazing valley ride and Vratsa's a really nice town.
