Speaking generically (not AA in particular), I travel with a backpack that fits my laptop and everything I typically need for three or four days away. And it fits under the seat easily, every time. If I do put it in the overheads, it fits very nicely into the curve at the back of the bin, so usually something else can go in front. If I have anything more, always checked.
It does mean that I usually don't have to worry too much about getting in early for the overhead space (unless I score an exit row; no bulkheads for the likes of me), but it drives me up the wall having to wait for people spending 5 minutes squeezing in their luggage of all shapes, sizes, and quantity - and then pushing around looking for where they put it at the end of the flight.
Honestly, if it needs wheels, it really shouldn't be being carried on.