I regularly use 10-15 metre runs of cat5 cable between PICs. With a MAX232 interface chip I can get 115,200 bps at 15+ metres and at TTL I don't know the maximum because I have not had to go over 19,200 bps.

I find that the voltage drop in the pair(s) used for power means that I have to run 9+ volts and have a regulator at each remote PIC.

I use three wires for GND, 3 wires for PWR and the remaining two for data.

I wire these as
1 pair - PWR and PWR
1 pair - PWR and TxD
1 pair - GND and RxD
1 pair - GND and GND

I always use a checksum on every message passed and flash a LED when an error is detected. From the occasional flash of the error LED there are real world noise issues as with every form of serial comms beyond the PCB.