The more I play with it, the less I like it. The concept is nice and all, but it’s too inflexible.
If you want to build a quick and dirty, generic web application for mobile devices, then by all means go ahead and use jQuery Mobile.
If you want to build a pretty brand-specific website, don’t bother.
You can do away with the page transitions and dialogs that jQuery Mobile gives you and style that all up to your liking without the overhead of the framework, and without the headache of battling against its bulky CSS.