It seems like most people are not enthusiastic about subscribing to the AT&T plan after the free trial expires, however for me this is pretty attractive so I thought I'd share my reasoning to add some diversity to the discussion.
The data plan allows for the WiFi hot spot to be activated which is great for when I am shuttling kids around through slow and boring Bay Area traffic, so they can watch a show or do their homework on their iPad. It also allows me to work from my car on my laptop while I am parked and waiting for kids.
It is true I could also use my phone to generate a WiFi hotspot, but to activate that feature on my phone I would have to pay the same extra monthly fee and I would have to turn it on/off all the time and it would drain my phone's battery, and I am guessing the connectivity of the car with its external GSM antenna is better than the phone in my pocket inside this aluminium cage.
The cost of $20/month is of course a consideration but I am lucky that my employer pays my AT&T bill since I use my phone and this WiFi hotspot function for work.
So I think this is a very nice and convenient feature to have.
(My only slight problem is that I can only get a 3G connection and AT&T tells me the car can't do 4G/LTE but from reading posts on this forum this isn't true. Fortunately even 3G seems fine for streaming Netflix and doing whatever I need to do)