I've done the GA 3 times, all for arena shows. The first was in Atlanta on the Vertigo tour, and we got there at like 10am, waiting all day. It was a fun experience, though cold. We made friends w some people who were in line w us, and we did get a good floor spot, though not on the rail.
The other two were on the I&E tour, in Belfast and Dublin. For both shows, I made no rush to show up "early", and, while I obviously didn't get a rail spot, my spot was quite good for both.
I'm doing the GA for Pittsburgh in June, and, as the stadium is so much bigger, I'm not planning on arriving crazy early. When U2 played Heinz field back in 2011, friends of mine didn't go in until super late, and still managed to get inside of the inner circle. At present, according to Ticketmaster, there are still a solid 3,000 GA tickets available for Pittsburgh, so we'll see how that ultimately pans out come June 7. I'm confident that most, if not all, will sell, it still is kinda surprising that so many are still available. With that said, I crunched the numbers, and they've still sold about 90% of the tickets for that show!