Usually when you go to a larger diameter wheel, you also change to a wider and lower profile tire. This keeps the outer circumference about the same which keeps the speedometer relatively accurate. There are several possible issues with this:
1) Wider low profile tires generally put a wider patch of rubber on the road. This increased size of the contact area does generate more rolling friction which can adversely effect mileage.
2) Lower profile tires have less "give" when you hit a rock or pothole because there is less tire there. This can lead to more easily damaged wheels.
3) Lower profile tires are often more expensive to purchase. Check on a website like tirerack.com and compare prices. For the same kind of tire, the lower the aspect ratio - the higher the price.
There can be exceptions to these rules, but they are things that you should be made aware of. If you are not going to be racing or autocrossing, the only advantage to bigger wheels is looks alone. If looks are more important to you than all of the above - then go for it.
Have Fun!
Don