Your putting in a new countertop in your kitchen and you always wanted an island counter. Here are a few things to consider.
Will an island fit? If you force an island into a kitchen where 
there was not one before it can make the kitchen less functional rather than more. You should have at least three feet between the island an all other cabinets or appliances to make it work.
If it fits, what do you intend to do with it? Is it a work space only or do you want to create knee space so you can add chairs and make it a breakfast area with high stools. This can be particularly beneficial if you entertain a lot and you guests seem to congregate in the kitchen.
How will you layout the cabinet? Make sure you place doors and drawer so they open in the most accessible direction.
Do you want to add plumbing such as a vegetable sink?
Most island today are designed with one or more convenience outlets. If you are using it as a work space and need an electrical appliance a chord running through space to an outlet by the wall can be a problem.
You should also consider building the island and installing rollers with position locks so you can move it out of the way when necessary. This idea sounds extreme but I have seen it used very effectively.