I travelled in the last week (on Frontier) and an announcement was made prior to boarding and on the aircraft that passengers should put their carryon in the bin and their 1 personal carryon under the seat in front of them. As one of the earlier boarders, I got to see how it all worked out for the rest of the passengers. In both of my flights, there were some people that tried to cram both of their carryons in the bins, others complied with the announcment and for a select few, they didn't have a carryon. All the bins were completely full long before the last passenger every left the gate area. It is amazing to see what people try to get on the plane. The gate agent even offered to gate-check anyone that didn't want to bring their rollerboard onto the aircraft. I am sure there were several (especially the later boarders) that had to gate check their bags.