I usually hand quilt. However, there are some cases when it is necessary to use the machine. The current Quilts for Kids project mentioned in another thread, for instance, requires the quilts be machine quilted. Those will be washed daily, I understand, and need to stand up to a lot of wear and tear. Having said that, I admit I will be honing my machine quilting skills as quilt them.
My reasoning for hand quilting - it relaxes me. :D