Abu Dhabi is a great medium to long term investment. Resales are slow as the market is just really taking shape. Its mostly investors piling in to it right now.
Abu Dhabi, in December, released a master plan - a vision of Abu Dhabi in 2030 which was done by top planners and experts worldwide. The plan goes down in to as much detail as to how the roads will be sign posted. It is going to be a much better, upmarket city then Dubai. They plan to out do Dubai but want to be seen to be classier. However, Abu Dhabi take their own time on matters ... partially because they are so much richer than Dubai (they own 94% of the oil in the UAE). Abu Dhabi also has the highest GDP per capita of any city in the world.

If your outlook on property investment is to hold for a few years (at least 2-3 yrs), then Abu Dhabi property will do pheonomenally well. It is like buying in Dubai 4-5 years ago however the entry level prices are much higher.