The middle of the semester is an excellent time to ask your students to think about, and comment on, their perspectives on the course and on your teaching. Unlike end-of-semester course evaluations, midterm evaluations offer you the opportunity not only to make adjustments in your teaching "mid-stream," but also to return to the class with thoughtful commentary on your students' feedback and suggestions. Asking students to provide feedback at mid-semester makes it clear that you take their ideas seriously and that you are dedicated to improving the course and their learning.
While you should always encourage students to talk with you if they have concerns about the course, students often need another, "low-stakes" means of communicating with you about those concerns. Moreover, providing students a chance to respond to the course, in an anonymous format, will encourage them to communicate responses that they may not feel comfortable providing in person--including positive comments.
Asking for student comments on midterm course evaluations may also help you to identify and address issues related to classroom dynamics that students might otherwise be reluctant to mention, such as issues that arise when other students are not fully participating in group work or are disrupting class by arriving late, talking to their peers, or surfing the internet on their laptops. Even when students do not address these issues themselves, midterm is an excellent time to reinforce course policies and expectations related to these issues--without "calling out" individual students.