Yup... self drive is more enjoyable(tired too of course).. Also own timing means you can get photo at the type of background and at any timing(sunset/ rise) you want....
Drive to milfoprd sound can be abit dangerous winy and steep road... long drive 12hrs so better to have sometime to take over from you when returning....
In my family, not one of us can drive so we took the coach service offered... Simple, easy and you get to cover most things.....
When you reach christchurch or any town.. look for this place call I place... They provide wonderful information and also help you book or organise tour....
lastly, ppl there are VERY FRIENDLY