If seems to me you have at least two target audiences. The main one seems to be professional QA Testers and/or web services developers. They probably have little problem "jumping right in" with the tool. Then there's the rest of us, who are more casual users. I'm an application developer working with an external (3rd party) team who are developing the web service. I just need a tool that allows me to use the service outside my own code so I can experiment before I start writing a bunch of code.
One suggestion would be a README to go with the Tutorial files. I figured out that I have to import one or both of the sample projects, but a few simple examples just typing in parameters in the request and seeing what comes back in the response would be great (especially if it included an error return).
I think SoapUI looks like a great tool, just give me a little help to get over the learning curve and I'll be much more likely to pay you a few $$$$ for the Pro version. Show me the value, and I'll show you the money.
Thanks,