I hate calling software "a mistake", but I can't find any other way to describe Gerrit. You may see people using Gerrit 'cause Google uses it. The thing is: Google misunderstood what Git actually is.
When Linus Torvalds came with Git, he was trying to mimic another system, BitKeeper. Along with some improvements over CVS and SubVersion, Git made really easy to create and merge branches, something that was either almost-not-supported or slow-as-heck, depending on which tool you look at.
You need to take this into consideration: Git made branches easy.
Then someone came with the idea of Gerrit: Instead of managing branches, it actually manages commits. Instead of having a branch for each feature, you should have one single commit as feature. You can have branches on your machine, but the server only deal with commits.
So Gerrit took Git, a tool that improved the way we deal with branches, and removed branches. This is akin to taking a text editor and taking away the ability to edit text. Does that sound right to you?
In my personal opinion, what they did was to take git apart and put an err in the middle: gERRit.
When I see someone using Gerrit, I know something is wrong there.