This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
I guess It is much faster to test my webservice using soap ui than to generate a client and write code in order to test it, so it accelerate my development.
However I use soap ui when I have interoperability between .net and java or mismatch between the wsdl and the stub generated, So it is important for me to check the message sent and received at the xml and soap level in order to check conformance.
You can also take a recorded request and introduce an error to test the service error responses. Deliberately creating errors with client software is quite difficult and impossible for some kinds of errors.