I left my unlimited plan for the $45/4GB hotspot plan. No regrets. With 3.0, you could tether with a USB cable which I did and made my laptop the WAP for my wifes iPad, but eventually had to upgrade killing the "free tethering".
I checked my data usage back as far as I had had the phone and had never used more than a few GB in a month so I switched. Now, last week, we were camping and DW was working from my Hoptspot and I got a 3GB warning from ATT but she was doing lot of uploads and downloads of pictures. BTW and additional GB is $10 if you go over so not the end of the world.
I believe you can tether with your unlimited plan IF you Jailbreak the phone but there are downsides to that.