I was wondering if you can substitute buttermilk for water when called for in a cake recipe? If so what are the taste / texture effects? I have never tried it but I have heard of people never using water.....

Are you substituting for allergy or lactose intolerence? or not having buttermilk on hand?
If it's for the second reason, you could use 1 cup milk add 1 tablespoon vinegar allow to stand for 10 minutes. The texture of the product is definitely better with buttermilk.

I have always just used water when the recipe calls for water but was wondering if i were to substitute buttermilk in if it would enhance the flavor / texture instead of just using water?

I think it will give you a moist crumb, and doesn't dry out when left a few days. The only way to know is to try it out. Some of the best recipes come out from trial and errors.lol

I use buttermilk in place of water when using cake mixes all the time. It works great!

I, too, will use buttermilk in a recipe that calls for water. I also add a small amount of baking soda to offset the acidity.