Depends. In general, I think it is a good thing to try the software and use it if it suites your needs and taste, even if it is young. Usage and a community will help the service grow and problems like that data leak will help Gitlab to become better in the future (because they will likely employ measurements to prevent something like this from happening in the future again).
However, you shouldn't use it for important stuff! But hey, your important stuff likely has more than one backup either way ;)