Here's a suprise for ya : many professional cake bakeries use cake mixes. I worked for one that used pillsbury.
The reason is, it allows for consistency in taste and appearance.
mixes are pretty darn good.
Unless you're going for something more complex, like an italian creme cake. Make it yourself or leave that up to the pros.
My current favorite cake is godiva's buttermilk chocolate cake. It's not overly sweet and very decadent.
http://www.godiva.com/recipes/recipe.asp?id=302
I have found that you can leave out the liquor if it is too expensive, and pretty much any frosting goes well with it.