I guess all has been said which can be said about Nessebar. The small city and its wonderful unesco churches are without any doubt worth a visit.
I stayed one night in Nessebar on the 14th of jan. Very few visitors were there,hardly anyone after sunset.
I had the town to myself in the later hours of the day. The locals told me that Nessebar is a pure hell in the summer months and even in spring as the city is virtually overrun by hordes of tourists from the nearby golden beach + local bulgarian tourists from other parts of the country.
I managed to see 2 unesco churches from inside and visited all places of interest in that small bulgarian town. Most churches were closed in winter time.