It's definitely better at Sofia airport with the regulated taxis and although sometimes you might get a taxi driver either taking a longer route or asking for a higher fixed price, in general I've found ok taxis to be fine. You still might be approached by more expensive taxi drivers but just ignore them.
taxis in Bulgaria have their name and licence number displayed on the dashboard so if you feel you are being cheated, write it down and ask for a receipt so you have some comeback.