This episode is rarely mentioned in "best TZ episodes" polls, I suspect because it has none of the thoughtful social commentary Serling is famous for. Still, for what it is, a straightforward horror story, it's quite effective. There are a few scenes that remain creepy however many times you see them, and if you don't see the ending coming, it'll likely deliver a memorable shock. I didn't predict the ending when I saw it (age 11 or so?), and when ventriloquist-Willie turned to smile that very creepy smile at the camera, I was so freaked out that my heart skipped a beat (I mean that literally).
All that said, I am not sure how well this episode holds up to repeated viewing. It surely remains unsettling to people who fear ventriloquist dummies, or can put themselves mentally in Jerry's shoes (pretty scary, that). For most everyone else, though, I imagine once the ending is revealed, the thrill is gone--and that thrill is, by far, the best part of this episode. And it still bugs me that he locked Willie in the trunk instead of taking him out and destroying him right away. It's not as if it's critical to the story; Willie could easily have returned afterward regardless of Jerry's attempts to destroy him. (RS did adapt this episode from an unpublished story, so that may have been an element of that story that RS didn't want to change.)