For me at least, the $100/year is quickly made up in those instances when I need overnight shipping. Prime gets you free 2 day and really cheap ($5 or so) overnight.
Having some random part break and be here the next day is a godsend.
If you need it then I can see where it would be totally worth it. Personally, I'm not sure I've ever had something to order online where I needed it overnight.
Having some random part break and be here the next day is a godsend.