This paper presents an analysis of the user experience of Reframer, a novel human-AI drawing interface designed with the iterative and reflective nature of creativity in mind. Collaboration with Reframer occurs in real time, with the user and the system drawing together concurrently. This approach is inspired by theories of creativity as being more problem-framing than problem-solving, and contrasts with the automated one-shot end-to-end workflows of most generative AI models. A 12-participant qualitative exploratory study of the capabilities of our prototype is detailed, as well as a thematic analysis of user attitudes towards drawing with it. The paper then describes two modified prototypes and a second 32-participant comparative study revealing how interface variations evoke differences in user attitudes and experiences. It concludes by proposing a model that characterises the conditions under which users experience co-creative AI as a collaborator, rather than a non-agentive tool.