Thoughtworks has used jQuery Mobile on two projects with mobile websites and had mixed experiences. One project found the library very useful for dealing with device differences and graceful degradation on older browsers. On this project we were working in a way that fit with the jQuery Mobile approach. Our other project found the tool less useful and felt to some extent it was trying to force them to work a particular way that did not fit their application well. For these reasons we have decided to leave this tool in assess. If you are doing mobile web it is definitely worth spiking but it may not fit every application.