I’ve almost finished a prototype for work and have set up a nifty little testing studio with Magitest, Reflector and Invision. With this setup I can put my prototype in the user’s hands and let them play with it, and Magitest will record all their interactions with the prototype, as well as their reactions and comments.
I’ve also spent a fair bit of time running through the testing script, to ensure I get useful feedback. As Nathanael notes, just putting a prototype in front of a test candidate is highly likely to result in artificially positive results.
The problem with sticking a prototype in front of users without any idea of the success criteria is that there is a very good chance the prototype evaluation will be a success. Why? Because you invite people to participate in the activity, probably pay them for their time, put them in front of something that you’ve probably worked on (or they assume you have) and they’re more than likely to say that they like it, that it’s good and yes they would probably use the actual product.