I use wireless charging almost exclusively for my S5 and before that with a S3. I bought the charging coil that goes behind the original cover so I can still use any case I want. I have 3 charging pads. 1 by my bed, 1 on my desk at home, and 1 on my desk at work. Yes it charges slower, so I think it would be pointless in the car. I'd be surprised if it could even keep up with using the GPS. But, like now while my phone is sitting on my desk, it's great. If I'm setting my phone down while I use my PC or sleeping it might as well be charging and with no wear and tear on the micro-sd port. So, it's good for keeping your phone charged when you're not using it much but it's no replacement for the charge rate of a corded charger.
edit: Also as far as heat, I haven't had any problems with batteries going bad. I got over 2 years out of the original battery in my S3 and I'm still using my original battery in my S5 which is just over a year old.