With the recent development of artificial intelligence (AI) technology, services such as automatic writing evaluation (AWE) and AI-assisted writing have been developed. AWE is often used in education to correct grammar. Unlike AWE, AI-assisted writing is a service that generates content for users. Currently, being a new field, research on AI-assisted writing is insufficient compared to studies on AWE. This study derives user experience factors of AI-assisted writing through a literature review and checks the difference in experience according to the writing case and creativity level. Seven user experience factors were derived through literature research, and the factors were evaluated for "Rytr," an assisted writing service. Thirty-six subjects participated in an online survey. Four prototypes were designed and provided according to the writing case (e-mail, story plot) and creativity level (high, low) of text generation, and a two-way ANOVA was conducted based on survey data. A significant difference was found for each creativity level and writing case. Users obtained more creative writing results when writing a story plot and thought positively about the assisted writing service. In the AI-assisted writing service, the users experienced increased trust and satisfaction when writing a story plot. This study can help design functions based on various types of writing, considering user experience in developing AI-assisted writing services in the future.