I have a subscription. It works out to be about a 50% savings against a non-subscription. My average charge at a Supercharger cost almost exactly $20. A savings of $10 on average per session means a $13/mo subscription fee is recovered in about two months. The average cost for my use of superchargers is about $0.42/kWh, which is quite high and gives me an average cost/mile of about $0.17/mile, which is pretty high.Generally speaking, yes. Rates at Superchargers are higher for non-Teslas. One can create a Supercharger subscription account, which gives lower rates, bit you'd need to do some math to determine whether the cost of the subscription saves you anything in the end. Basically, the ROI tends to only turn out to be worth it if you do a lot of DC fast charging at Superchargers. I think it's possible to easily create and cancel the subscription, so that approach might make sense if a cross country trip is in the cards.
For non-brand-T fast charging, my average cost per kWh is $0.36/kWh, and about $0.15/mile, which is also high--about as high as I would be paying for fuel for a 4x4 F-150 ICE with 10,000 pound towing capacity.
When I charge at home, I pay about $0.02-$0.07/mile, depending on various factors like whether I force a charge during peak times or charge during super off-peak times.