Plan the height of your cabinet appropriately. Since most islands will have at least two stools or chairs pulled up to the surface, you may want to think about the seating or stools first. There are two kinds of bar stools with one being much higher than the other. Unless the island will be used primarily for family eating, there really are no hard and fast rules on height. It does not have to match the height of the kitchen cabinet, but often does. If it is not going to match it should be taller in most cases.
Safety First! You are better off without it if you are using up all the free space in the room. Your kitchen should be spacious and you should never feel restricted in your movement - in fact it can be dangerous if you are. Yes, a kitchen island is good to have, but not if your safety and freedom of movement are compromised to accommodate it.
A kitchen island can be a good option for those who live in a small home or apartment where there is no dining room available. If you also plan to use this furniture piece as a dining area, make sure that the seating is comfortable enough for eating.
If you are looking to do a complete kitchen remodeling for yourself, or for maximum resale value, don't neglect the kitchen island. Once just a place to pull up some stools so the kids could eat before running to the bus, the island and it's function in the kitchen has changed quite a bit in the last 10 years.