I think up until last December, the Mini's were essentially free (you'd buy them for $10 but get a $10 credit) now you have to actually pay $10 for them. That said, the convenience alone makes it worth it although we pretty much only use it when driving through Orlando. We don't use ours tons so ours automatically refills to the minimum amount (I think it might add $10 whenever we get under $10)
The minis are designed to only be used for one car but I've only pealed the corners off not the whole sticky backing and was able to switch it or remove it from the vehicle. We have one in our mh and one in the tow vehicle and if you don't remove it, you will be double-charged through tolls (although you just have to call them and they'll correct it)
As far as the post above asking if it can be used WV, VA, PA and NY, I don't believe it can. I believe those use EZ Pass not SunPass and I don't think they are compatible.