We started seriously looking into git in 2007/2008, then launched git.kde.org in 2010. We had set ourselves the goal to lose zero history, and doing a perfect import while transitioning from a monorepo (with lots of interesting code movement within the hierarchy) to hundreds of individual repositories was quite challenging back then! We had to invent some of the tooling (svn2git) along the way.
We're pretty confident it had a positive impact overall. Our repo boundaries match our source archive release deliverables (source tarballs) since the switch. The consistent chunking, also in the build system (i.e. components finding each other during build - before mostly packagers were exposed to this, post-switch every dev is implicitly), helped us along in splitting/modularizing/tiering especially our libraries. It's made us more conscious of dependencies and made it easier to swap things out to rejuvenate.
The community has seen some challenges with drifting apart along the way (projects isolating vs. working on common themes/initiatives), but I think this was more a side effect of overall growth than just the repo reorg. We've also since combatted it with the very successful community-wide KDE Goals election process.
We've stuck to the same ACL approach throughout, notably - you gain KDE dev rights through a peer-support process based in merit, and once you have them you can push to any repo. We mandate code review through strongly enforced social etiquette, not by ACLs.
