Has anyone tried the Fossil SCM? What's your opinion?

Git was designed to support Linux development. Fossil was designed to support SQLite development. Those projects have very different needs. The details are the subject of a 1-hour talk and are more than can be compressed into a HN comment. Sufficient it to say that (I believe) there is room in the world for more than one VCS.

Earlier today, a support customer needed to know all SQLite check-ins between versions 3.8.7 and 3.8.8.1 that touched any of the files src/pager.c, src/os_unix.c, or src/wal.c. The answer is on this link: https://www.sqlite.org/src/timeline?from=version-3.8.7&to=ve...

Is a query like that even possible with Git or GitHub?

In fairness, I had to enhance Fossil slightly in order to support the query above. But the enhancement was minor and only took a few minutes. See the diff at https://www.fossil-scm.org/fossil/info/b2b62b8318700f9f

However, I eventually switched to git, so that I may easily push to various remote publish repositories such as gitlab and github.

It's a nice feature to have everything integrated into fossil (issue tracking, wiki, etc). However, it is also a drawback or waste when you actually want or need to integrate with existing infrastructure and diverse services providing those features.

The world has standardized on git, and there's a whole ecosystem of tools and services developed around git. So unless you have a very specific and strong reason not to use git, it's hard to make a case for any other DVCS.