I recently flew SAN-JFK-PSE-JFK-SAN and experienced all three. In SAN it was all at the same time. At JFK back to front and from PSE to JFK dual boarding and I actually boarded the plane from the back, which was a first for me. From JFK back to SAN it was again back to front. The flight from SAN to JFK was not that full so it wasn't a big deal boarding all at once, however, I can see some issues with boarding all at once with a full load. One thing that bothers me is that when the gate agent announces "now boarding rows 19 to 26" all the passengers on rows 1-18 stay right next to the entrance creating an even a bigger mess! I can see people wanting to get overhead space but we're all boarding the same plane! To each his own, but that's why I usually fly with bags that fit under the seat, everything else gets checked in. It makes my flying experience a bit less stressful.
Just my .02