This research explores the impact of ChatGPT, an AI tool, on ESL students' writing proficiency at a Saudi Arabian university. Using a mixed-methods approach, data from 130 students showed a significant positive effect on their writing. A focus group interview with six students highlighted opportunities such as personalized feedback, language skill development, vocabulary expansion, and greater autonomy. However, challenges included a lack of contextual understanding, loss of individuality in writing, reduced creativity, and ethical concerns. The findings offer valuable insights for educators on how to effectively integrate ChatGPT into writing instruction while addressing its challenges.