Where you have to be careful is on itineraries in which you know you will be on a RJ or smaller aircraft. I just completed a trip which included a leg on an EMB120. If you are in the A seat you couldn't fit much underneath the seat due to the curving of the aircraft taking up some of the space. Knowing I would be on this leg I use my Orion AW which splits into two. Both pieces fit in the overhead without any issues and my laptop bag went under the seat. I just make sure I reattach prior to leaving the plane so I still have two pieces. I had two bodies and 4 pro lenses in the belt pack so it does hold a good amount.

If I would have done this with the Lowepro PhotoTrekker I may have been stuck doing a plane side check which isn't going to happen.