Has anyone checked a backpack on a plane?

Yes, I’ve seen it done all the time.

I have been at airports and have seen people doing that,maybe call the airlines and simply ask.

I checked a large pack pact a few times going to Eastern Europe, Australia and New Caledonia. Had no problem.

I’ve had no problem at all carrying a rucksack on a plane either. There are weight restrictions and theoretically size limits as well but I’ve never had a rucksack checked for size.

Yes. It is no problem! Remember odds are a suitcase or two will land on it so don’t put anything fragile inside.
And be sure to tuck the straps in where possible.

Should be fine. I do it all the time. They will want you to take out your laptop/iPad and put it in a separate bin. Remove all sharp and pointy objects (hijacking tools). Don’t bring any containers bigger than 3.4 oz, except for an empty water bottle.

@RocketGuy The OP is talking about checking it, not carry on.

I see. Don’t put anything valuable in it, then. Bags frequently get opened up unless locked.