We had a more complex GUI that supported OR-ing multiple AND-ed panels together, but it was too unwieldy and complex. Two things we did to address this is that usually when a user wants to OR, they want to OR multiple values for the same field, so we allow doing something like city=toronto,ottawa,"new york" which is the same as city=toronto OR city=ottawa OR city="new york". Also, we support a more complete query language in the text field, which supports AND/OR/NOT/()'s. As they perform searches with the GUI it shows them the equivalent query so they learn the query language, and then all they have to do is add OR's if they want to combine them. However, the OR in the query language was broken for many months at one time, and nobody complained, which leads me to believe that nobody uses that feature.