It is definitely not invasive w/r/t your service beans. However, working with your services directly from the client implies (at more than one level) that you are actually building a whole fat-client-style functionality into the HTML/JavaScript. As opposed to AJAXTags or Taconite where even your presentation layer can remain mosty unchanged from a vanilla HTML/Struts app.

DWR is might cool, but if you are attempting to retrofit an application with AJAX functionality, extensive used of DWR does require a rethinking of your presentation layer.