So far very unconvincing replies... Here is my take on this...
Islamabad has become a large city in the past decades. The original Islamabad that was developped up till 90's is a small section of the city while the city has pretty much taken over Rwalpindi from all sides.
There are too many good locations in Islamabad but the larger developments would be Bahria, Defense Housing Society (DHS), Naval Housing, Various sectors of Islamabad (F6,F7,F8,F10, G6 to G10, etc.). I feel the real Islamabad culture remains between the old sectors (F & G) but many would not agree as those are overly expensive and out of reach besides newer areas have some to offer.
Cost of living is fairly cheap as compared to most other cities with similar offerings. Islamabad is about 30% cheaper than Istanbul. People of Pakistan are probably the most hospitable folks in the world. You introduce yourself as a tourist and 10 families would offer to host you or pay for your dinner.
Islamabad is one of the cleanest and naturally beautiful city. Most of the city is well planned but you'll encounter some older villages within the city that got incorporated into Islamabad and still have crocked streets and whatever that comes with being ancient.
Realestate is always the best investment in Pakistan as population has been growing and land is limited. This implies that realestate is not going to be cheap. Once you have a housing the rest is ultra cheap though. The cheapest apartment may start at about $35000 to $100,000 while houses range from $150,000 to $600,000 depending upon the size and location. Sure there are always some locations that can be in millions of $ but we are only talking livable housing.
Let me know if you need any further information. I can try to answer any questions. I have not been to Pakistan in a while and some of the information may be out dated but I can get the latest prices etc if someone in interested.