The stock is worthless. They are in bankruptcy! The creditors have taken control of the company from the shareholders and will distribute the value as they see fit, up to the point that creditors receive 100% of the money they are owed, with interest and penalties, which will not happen in our lifetimes. Therefore, the shareholders will eventually have their shares cancelled once the creditors decide to sell the company, either to another company, or by sending it public. Given United's current situation and the quagmire of poor management and organized labor, I would expect them to liquidate in the next 5 years ala Eastern.
If you want some cool certificates, then buy the stock, if you want an investment, you might look for a company that has a good long-term outlook, which United does not.