Over the past several years, I've found more airlines are reluctant to allow me to bring my guitar aboard in a gig bag. I always carry copies of the Musicians' Union / Airlines agreement that explicitly allows guitars in gig bags to be carried aboard, and to be stowed in the overhead bins. However, this is always up to the discretion of the airlines based on available space. I frequently am told "there is not enough room in the bin for your instrument", even when I'm boarding early (thank you, 75K mile gold club!) and the bins are empty. On those happy occasions when I have successfully stowed my guitar in the bin, someone will still attempt to cram a suitcase in on top of it. When I am denied taking my guitar aboard, I have been required to check it - with its only protection being the gig bag! I've been told by some airlines that I should pay for a seat for my guitar, which I know is actually done by musicians who travel with valuable instruments.
A few years ago I started to travel with my guitar packed in one of those SKB 3i cases, and checked as luggage. The luggage fee is a drag, but it's much cheaper than buying my guitar seat! Those cases are Godzilla-proof. They need to be. Baggage handlers treat it just like a suitcase : they stand it up on end on the conveyor belt, and just let go, allowing it flop onto its face. At least I can be reasonably assured that my guitar will arrive in playable condition. I've been happier with the SKB 3i cases than with my traditional anvil-style flight cases. They're much lighter, and much easier to manage.