Thanks Sophigirl and Smartshopper for the additional information and tips..
We will be in Europe (Vienna, Budapest and Prague) for a total of 11 nights in September. I think I have pretty decent rates but after seeing some of the rates in this thread, I thought it might be worth spending some time looking for a better rate. It seems that several of the 'codes available for public use" do not apply to European hotels. The ones that were eligible did not produce a better rate than I already have. So the consolation is that the rates found are reasonable..
I became a 'gold' this year, so getting access to lounges and possible upgrades are great bonuses.
We will be in Boston for a couple nights also in late September so will try to use the bogo cert. for that...