We have used this cover in sizes newborn, small, and medium. The velcro is strong and we get a better fit on her than many of our other covers. It's a bit roomier than our other covers so the small did not fit at the lower weight range of 8 pounds. The newborn cover may have not been necessary since she wore it for such a short time, but it was much easier to avoid the umbilical cord with the newborn cover in the middle of the night. Of course, the inner leg gussets are a must.
Update - 5 months: This became the perfect cover for a while. Small enough to not leak over prefolds but roomy enough to fit over her bulky night time diaper.
Update - 8 months: We bought 2 mediums since we preferred sized covers. They work well over her bulky night time diaper, but are too big to go over just a prefold during the day.