I agree with all the latest posts...
When not carrying a backpack or a small (and unstructured) duffel bag, my wheeled "carry on" is virtually on the size limit. And it will have to fit sideways in old-style bins (pretty much everything domestic save for the 739s).
I just had a conversation with someone about how I hate to be the space hoarder or aisle blocker, so if carrying that bag, I will check it. It won't cost me anything but 10-15 minutes of my time, and it will save me (and perhaps my fellow travelers) a headache. Baggage priority seems to work most of the times in ATL and LGA (my most used destinations).
It does seem ironic that Southwest's process is indeed more civilized than pretty much all of the the legacies. I wonder if charging for carry ons could really be a solution (and I can't wait for the howls here).