Depending on what it is you are testing, absolutely.
With respect to user testing a website:
1) Quickly create a website prototype and place it in the hands of a user for feedback
2) Iterate on this feedback to change components/interactions/look and feel until you get the design right.
3) Observe user interactions with a website, and note points of pain/ease to revisit/redesign.
Does this answer your question? Let me know.
Thank you very much for your reply, I am working on a prototype right now! With the little I know about Catalyst I already feel that it may play a major role in the future of user testing/interaction!