I would just like to add our recommendation of Sikon Group (http://www.sikongroup.com
) in Sofia since using them in July/August.
When we originally booked a few months ago via email we got very prompt and helpful advice from Yasen, who runs the company with his business partner (whose name I have forgotten, sorry!). Yasen even pointed out that I had asked to book the car for longer than I actually needed!
Our flight arrived in Sofia at a horrible time in the morning (about 1.30am) and Yasen met us from the flight and gave us a full run-down of the car and provided maps, driving directions to get on the Varna road from the airport, which is a bit tricky, and some very useful advice about driving in Bulgaria.
During our stay we were involved in a very minor prang with a Bulgarian driver, who had scraped by us on a very narrow road but decided it was our fault, and rang Yasen, who sorted it all out over the phone.
On the way home we dropped the car off at an even more awful time of the morning (around 4am) and Yasen and his partner were there to meet us, smiling and friendly.
The car was very reasonably priced and was absolutely no problem at all. I can't recommend Sikon group enough, they are great value and very helpful, friendly and professional.