For enterprise projects this often takes the form of a pilot or an evaluation. For consumer projects it might be a beta test or simply sending it out into the world and letting someone download it. In either case, the feedback's gonna fly. There are many things you can do with this feedback and many responses that are completely legitimate. There's one thing you shouldn't do:
Try not to sound surprised when your "easy-to-use" software is actually easy to use.
If it fails, you can deal with it (how to respond is another post, but not uncommon). If it works, though, you want to inspire confidence. Being surprised that your software meets the claims you made does not inspire confidence.
Please avoid phrasing like, "I'm amazed how quickly you got it running" or "Oh, that's great; I'm relieved." If you accept that the claims you made have borne out, then your customers will be more confident, too.