EWR is only served by COEX from GSP. One in the morning, one around noon and another about 6pm. They're all 50 seat ERJs. When I was a supervisor for COEX their, the morning flight was usually about 3/4 full, the noon about 1/2 full and the 6pm usually was close to full...if not overbooked. Sundays, Mondays and Fridays were always the busiest days. There are a few guys (some on this forum) who commute from GSP to EWR and IAH on COEX...who might be able to help you out more. The parking garage at GSP is now $9 a day and remote is $7. Personally, I'd only use GSP as a backup considering how many more flights to EWR there are out of ATL (on CAL and DAL).
BritPilot, any insight?
Cheers