1. Technical/Usability feedback
Call me stupid, but why over-engineer something that (usually) is done as a simple one-page form?
Slow, multi-page wizard, although a good looking one, is an overkill for a financial calculator IMO. I am on a broadband connection, I hate seeing "loading..." splash screens. What is being loaded? A form with 3 combo boxes? In fact the entire thing can easily be done on one page with JavaScript showing me results live - as I type. This way tweaking stuff is easier, more interactive and engaging.
2. Design/UI
Nicely done. Although, again, at expense of your own development time and usability
3. Usefulness
Well... I just turned 30 and retirement is kind of a depressing thing to think of... :-) There is definitely a demand for this, but I am not familiar with your market to compare you to the competition.