Lower Level seating on an island isn't common but it's something I personally think looks great and is very convenient. When someone is cooking and cutting up food, the guests or family are seated lower than the cook. Leaving them in sight versus higher seating where you constantly have to look up to communicate with them and away from what they are doing. Another plus to this arrangement is that you don't have to walk back and forth to a table with the food and can just hand it right over. To top it off these islands are more unique and can give an interesting focal point to any kitchen. This set up can be done in a very modern fashion or a more traditional kitchen as seen below.
This low island has a very modern feel
Lower seating in a more traditional kitchen
Look at these beautiful chairs!
Such a great smaller kitchen set up