After having used Fossil for more than a year now on non-trivial development projects, I feel confident enough to wage in on this topic.
Below's my experience so far. I'm comparing against git and svn at times, simply because I know those SCM's very well and comparing makes it easier for me to get the idea across.
I'm totally in love with this SCM, so it's mostly points on the pluss side.
What I like about Fossil:
1) We have a bunch of machines (win/mac/a number of linux distros), and the single-executable installation is just as beautiful as it sounds. No dependencies; it just works. Git is a messy pile of files and the dependency hell in Subversion makes it very nasty on some Linux distributions, especially if you must build it yourself.
2) The default Fossil workflow suits our projects perfectly, and more git'ish worksflows are possible when needed.
3) We've found it extremely robust, even on large projects. I wouldn't expect anything else from the guys who wrote sqlite. No crashes, no corruption, no funny business.
4) I'm actually very, very happy with performance. Not as fast as git on huge trees, but not much slower either. I make up any lost time by not having to consult the documentation every other command, as is the case with git.
5) The fact that there's a tried'n'true transactional database behind every operation makes me sleep better at night. Yes, we've been through more than one horrible incident of stale and corrupt Subversion repositories (thankfully, a helpful community helped us fix them.) I can't imagine that happening in Fossil. Even Subversion 1.7.x use Sqlite now for metadata storage. (Try turning off power in the midst of a git commit - it'll leave a corrupt repos!)
6) The integrated issue tracker and wiki are optional, obviously, but very handy as it's always there - no installation required. I wish the issue tracker had some more features though, but hey - it's an SCM.
7) The builtin server and web gui is simply brilliant and quite configurable through css.
8) We sometimes need to import to and from git and subversion repositories. This is a no-brainer in Fossil.
9) Single file respository. No '.svn' directories all over the place.
What I miss in / dislike about Fossil:
1) Someone please write TortoiseFossil for our non-technical Windows users :)
2) The community isn't that large yet, so it's probably hard for a lot of people to introduce it in their company. Hopefully this will change, gaining all the benefits of a large community (documentation, more testing of new releases, etc)
3) I wish the local web ui had a search feature (including searching for file content).
4) Fewer merge options than in git (though the Fossil workflow makes merging less likely to occur in the first place.)
I hope everyone gives Fossil a run - the world is a better place with stuff that just works and which you don't need to be a rocket scientist to use.
