aiContactSafe has a very logical structure, easy to use and functionally close to some of the most expensive commercial products.
Forms could be used "as-is" or embedded within an article or a module, most field types supported (file upload is absolutely "a must" for me). Validation rules and style sheets are customizable. What more do you need?
The support is excellent, I had to contact the author twice in the last 6 months and both times he was very helpful, replying literally within an hour and walking through problems with me (those were actually hosting-related problems).
I can recommend this extension to anyone without hesitation. Great product!