The out-of-the-box approval workflow is awesome and the majority of workflows in my experience tend to be some form of approval. The number one frustration with the previous version (2007) of the approval workflow was that it was not customizable. One small change request, no matter how trivial, meant creating an approval type workflow entirely from scratch. In SharePoint 2010, you are 100 percent able to leverage the out-of-the-box approval workflow using SharePoint Designer 2010!
If you have been jaded in the past by turning to the previous version of SharePoint Designer and its severe limitations on workflow development, I encourage you to look again! SharePoint Designer 2010 is a spectacular leap forward in SharePoint 2010 workflow development capabilities!
Now, I recommend you never, ever "touch" something provided out-of-the-box, so you will be directed first to make a copy of the out-of-the-box approval workflow...