What a horror story! You have every right to complain.
Do you know that you may be entitled to a refund for the baggage fees? The DOT has released new regulations which, under certain conditions, the airline must refund your baggage fees when your bags are lost. Here is the press release from DOT:
http://www.dot.gov/affairs/2011/dot11111.html
I think you deserve more than just the $100 refund. Have you had a chance to contact Customer Relations at United? They may reply and offer you additional compensation, such as a voucher. Good luck and keep us posted on the outcome.