lol @ OP. They messed up once, shit. Go back and complain to get another one if you care enough.
When I go to Starbucks, I usually just get a venti ice coffee with non fat milk and sugar. Taste fine to me, but I'm no coffee expert/snob
I usually go to McD for their ice coffees though, but they put hella ice in the cup, so you get less coffee.

And yes, it is usually sweet.