I haven't experienced any downtime with their public service - which I think is great for hosting private repos (as others have mentioned). I actually prefer it to bitbucket.
In other professional experiences, hosting our own instances of Gitlab was a great solution. Setup/updates were easy and we had the piece of mind knowing that we were in control of the maintaining uptime. I was very impressed by how simple it was to their implement CI service as well.
Just this past friday their fully automated CI devops whatever deployed an old major version to some parts of their infrastructure, the entire website was 500 for an hour and git pushes were barking up Ruby stacktraces.
GitLab.com is not deployed using any automated CI software, instead it's deployed using https://gitlab.com/gitlab-org/takeoff. The problem here was an incorrect version set in Chef, resulting in it deploying an old version to the cluster.
All the more reason to host your own instance! However, I understand if others prefer not to go this route and thus use an alternative version control solution.
In other professional experiences, hosting our own instances of Gitlab was a great solution. Setup/updates were easy and we had the piece of mind knowing that we were in control of the maintaining uptime. I was very impressed by how simple it was to their implement CI service as well.