I've been reading this forum for years, but felt the need to register when I saw my home town in a thread name!
They are supposedly the two nicest areas of Stockton (people from Yarm often claim is isn't Stockton or Teesside, but 'North Yorkshire'!). However, many people in Yarm have a highly inflated opinion of themselves - think football WAGs. Also, the pubs and bars of Yarm High Street are frequented by all of the scum from the Ouse to the Tyne. People who actually live there are most put-out by this.
Eaglescliffe is more of a residential area: there is no 'center' to speak of. Hartburn is very similar.
Darlington is very mixed. There are some seriously swish parts, and horrible ghetto parts.