I love this widget. Great idea, clean, easy to use. I can see how it would be perfect for a lot of websites.
However, for my website I was not able to use it because 1) the tab covered up some of my sidbar content, 2) there is very little control where the tab is positioned, 3) could not place it in the menue bar or header area, and 4) it did not give me the option of placing only the contact form on a contact Page.
I still recommend others take a close look at it for their websites though. I will be reconsidering it for my future sites.