There's a thread about this in the NYC forum, but it's worth posting here too.
As a FYI, from 1 May 2014 until mid July (75 days) the Airtrain "monorail" at Newark Airport will be closed for urgent repairs. This will affect
- Land side transfers between Terminals A, B and C
- Transfers to/from the rental car stations and some parking lots
- The Newark Airport railway station, which will be closed.
During the closure, all these transfers will be handled by shuttle buses. Those wanting to use the train services (NJTransit, NE Corridor and Amtrak, including to/from NYC) will need to take a bus to Newark Penn station for the onward journey.
This could have a large effect on flight connections, especially for international to domestic, where you clear immigration and customs and then proceed land side to your onward flight's terminal. All non-United INTL arrivals (and some UA too) arrive at B, and all domestic flights use A or C.
Here's the relevant information, including the NYC forum thread.