Yes I have. I have even used the KF2 box to dial in via a modem to get online with a DH+ network.
In RSLinx you have to select "RS232 DF1 devices"
and then specify "device = 1772-KF2/1785-KE/SCANPort".
You then have to match the baudrate, stop bits etc. with the many DIP switches on the KF2 module.
When OK, in RSLinx you can then browse out from your PC via the KF2 module onto the DH+ network. If your PLC5/15 is connected to the DH+ network you can then go online.
It will work, but the KF2 is awfully slow.
For serious work consider a PCMK (PCMCIA card) or PKTX (PCI card).
edit: I dont think there will be a major difference between ser.B and ser.C with respect to going online with your PLC5/15