Living in Pakistan is great now. It is peace out there and If you goanna start living in Islamabad is really cool as the majority of people in here are well educated, established and well traveled as well.
As long as the Karachi is concerned, It is being changed each and every day so be careful a little while living in there, But if you are preffering, DHA (Defence Housing Authority), and/or Clifton are good areas and peaceful. Try not to be out of your accomodation after 10-pm and try to arrange a security guard at home if leaving for days time.
Lahore is cool as Islamabad and always a great city for local and/or for expatriates. But in the areas e.g. Bhatti Gate, Delhi Gate, Mochi Gate, Near the railway station, Lari Adda (Bus Stand), Be careful as being careless may cause some problems e,g. no money in your pocket etc or mobile snatching for example. Preffer Dha, Model Town, Johar town, Bahria town for living in there and are good peaceful areas.
So all you new guys out there (Expatriates), All the best and Good luck!