another question, is the MiFare cards?
Is it a what?
A MiFare card is a brand or type of smart card working at 13MHz, it is perhaps the most popular card of this frequency (13MHz). It has space to store data as I said before.
are these only usable with certain reader/writers?
Yes most cards need their own reader so that the physical interface and the software protocol it uses matches.
being able to read/write from far away and multiple cards are all 'very' attractive features.
Yes it is but it adds complexity and cost to a reader and I have not seen a reader that will read any card there are just too many and it is very easy for someone to invent their own protocol. I know I invented several myself when I designed RFID readers. Most 13MHz readers can read the ID number from many different cards but after that reading data is different as is writing data.