I always check bags (even if all I have is a a carry-on weighing 20 lb) because I can, and because I have less to schlep through lounges and such and I don't need to worry about fitting all my liquid cosmetics into a 1L bag. Obviously, I am lying down on the metaphorical train tracks but my experience has not been bad, in terms of getting the bags eventually. I have had delayed bags (1 to 5 days) three times I can recall, in the last 200 flights or so.
The bad experiences are two.
First, AC bag tracing phone number is misinformed, chronically lying, or both since they will say they don't know where it is, claim they have it, ask what it looks like, and say it will be there tomorrow all in consecutive breaths while the bag is actually sitting on my porch. The real time tracking of other airlines, let alone any courier, puts AC to shame. As a tangent from this point, they authorize the delivery people to just drop your bag without any securement or signature. Combined with not telling you that they have it and it's being delivered means that it could sit there not secured until it disappears a second time, for good. My house porch is above eye level from the sidewalk so not in complete plain view, but my apartment door is right on the main path to the pool and spa. They could leave it with the leasing office right next door, but they don't. I felt lucky it was still there when I opened my door.
Second, AC will not admit to knowing about the Montreal Convention and will try all kinds of gambits. The key is to (a) know your rights, and (b) do not ask for authorization. Buy what you NEED based on what you had packed in the missing bag and now do not have. After some pushback, I was reimbursed for a suit in my home base because what was in the bag was needed for my next trip. When it was time to check in for the next flight, I could not wait any longer for the missing bag to turn up, so I bought a suit comparable to what was in the bag. Getting what you need and trusting the Montreal Convention doesn't mean head to Rodeo Drive and kick up your heels to some maximum authorization. It means if you need underwear, get it. If you need a nice dress shirt because you are going to a meeting, get it. If you ask, and they say "no", now you have muddied your claim because now you have to explain why you acted contrary to what they told you.