I've seen podman advertised as a dropin docker replacement. how valid is that assertion?

It almost is, the first change you'd see is understanding that each container is a separate process and thus for it to auto start you'd need to generate systemd service files. podman has an autogenerator for this, so it is 'just' two extra commands on the terminal but something easy to miss when you are starting out.

Worked for me for a long time until I found the samba-ad image won't run on podman. Worked fine on the same machine with docker.

Everything else has been working as a drop-in replacement.

I use podman desktop on Windows for common development stuff as drop in replacement. I switched due to licensing as I guess universities do not fall under the licencing exceptions. I actually also use the docker CLI, particularly docker compose. I was motivated to do the same on one of our Debian vms, because I could more install podmap via standard apt sources, i hoped that it doesn't mess to that much with the IP stack and it is a bit closer to K8s which I still deemed as overkill. However, trying to install e.g. komo.do via podman compose was a total fail. Even after fixing socket locations, etc, I would see weird behavior. So yes, it is a clear 'almost'. However, the critical cases can become easily very frustrating. Again, on my windows laptop with WSL2 it works like a charm, but there I also do not run server deployments that need to work reliably out of the box.

I wasted my time trying podman and switched to colima. It's the only usable free alternative that's a true drop-in replacement for docker.

I almost never notice the difference. I know there are some edge cases that will get you, but I've never run into them.

Mostly valid. There are a few gotchas but for most use cases it is drop-in. I think I've run into issues with the podman socket, and there were some permissions problems I had getting games-on-whales setup in userspace.

I had a brief period of setup pain and then have never looked back, though have occasionally wished that feature parity came a little quicker. Podman is in many ways a delight, simpler in what it does in the general case and yet as powerful as docker or more.