I'm doing some development work on a web map that brings wms layers from a host of UK public wms providers. Part of the work is to provide an info tool and we're using OpenLayers getFeatureInfo control to do this.
The problem with working with so many external wms providers is the lack of consistency in info formats supported. A quick summary (below) shows that html and plain text are the most supported with ogc xml a reasonable third. So we'll be concentrating our support for these formats and hopefully, if time and money permits, to extend support to others. Not good findings for ogc gml as it comes last with only 11% support.
100% support: html & plain
89% support: ogc xml
67% support: xml
19% support: esri raw xml & esri feature info xml
11% support: ogc gml