They don't come with a fastener which could be a pain. I've never used mine when I've been working in clubs, my transmitter would probably fall off
As Silver Pixels has said, with the RF-602s you have a transmitter unit, and reciever units. They are different designs and requires at least 1 of each to work. Multiple recievers can be used with 1 transmitter (I often use 3 for studio flashes).
The RF-603 combines both the transmitter and receiver circuitry into 1 device. So as long as you have 2 or more 1 can be used as the transmitter and all others as receivers.
Beneifts are, if you for example, have 3, and 1 fails, you aren't potentially left without a transmitter or receiver, as you are with the 602s.
602s and 603s aren't interchangable, and as far as I know (happy to be corrected), won't work together.
A lot of people like and use the 602s, they are slightly older but very much a proven design