$100/year is nice but $0 is nicer. It's usually not hard to combine items to reach the free shipping threshold, and there are plenty of alternatives if that doesn't work.

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.

I don't buy enough in a year to justify it.

You also get Music, Video, Photos, Now and a bunch of other stuff. No-brainer at $100/year.

It's a no-brainer if their offering appeals to you. For me, it does not. I get the stuff they offer for free/cheap from other services, and they tend to have a better selection for my interests.