Hi all am new here. My group has been doing webservice testing for quite a while now (using non mercury products) and we met with the Mercury product team prior to the launch of their Mercury Service Test application. They picked our brains as we have a enterprise license with Mercury and have been doing this type of testing for such a long time...
Here are my 2 cents on the product: it is a stab at doing web service testing, but is not up to par for directly testing a large web service application... the best way to describe a good usage of the tool would be where one is testing a UI application that invokes webservice calls to it's app server... the MST product could be used to help capture the webservice calls that the application invokes and compare them against a baseline... if you were directly testing the webservice app server the product will fall well short. The version of the product I saw (pre-launch) was not usable at all... very confusing and no workflow.
My group uses Integra Enterprise from Solstice Software for our webservice and MQ testing, and SoapScope from Mindreef to a lesser extent.
So far I think MST may work pretty slick. Not many in Support know much about it, so you are stuck with the few individuals that know the product. I'm still working a couple issues, but I think it's better than SOATest which I'm currently using. More flexibly as far as data management and automation.