We have just installed a three transmitter simulcast system over IP using Harris Intraplex. Being involved in other simulcast systems I was very pessimistic about the ability for the timing to be constant but so far Its doing pretty good. The media it uses for transfer consists of Tsunami radio's, fiber routes, and some networking over microwave.
The system is in Sequim WA. The Fire Dept there will be using it soon. Were doing some final tests now.
The stations are MTR2000's. The clock and ctcss filters are normal off the shelf products. The radio's are within 1 Hz of each other. The system is narrow band.
edit= to clarify off the shelf statement.