What to Expect: Mixing Viagra and Modafinil?

If I take modafinil and Viagra, what can I expect? I know there can be minor interaction and some side effects, but what are some of the positive results after mixing it?
1. If I have ED and take both Viagra and modafinil, what can I expect? Any positive results?
2. If I don't have ED and mix both drugs, what can I expect?

A detailed discussion is appreciated. Thank you.