I have been fortunate enough to be a part of some of the on-site player tests that we've hosted here at the studio and I can honestly say it has been a tremendous help. So if you happen to be in the area and have some time to do it we would really appreciate any feedback.
As a UI Programmer, having players come on site gives me an opportunity to observe where UI may or may not be working and hear first hand what their suggestions are for what features they might want to see. It is also great to be able to ask them questions on the fly that the discussions bring up.
Great idea! My husband works with programming, he has to take care of quite a bit of UI work for their product. Every now and then he gets to visit the companies that use their programs and he always mentioned how good it was to see in real life how people used the product and that different questions come up when you are talking face to face from what you hear when people would have to mail in questions. Wish I could come over and help myself, it is always interesting to discuss how programs work.
Good luck and I hope it turns out well.