Julien Danjou, the project technical lead for the OpenStack Ceilometer project, had some choice words to say about github pull requests, which resonates very strongly with me:
The pull-request system looks like an incredible easy way to contribute to any project hosted on Github. You're a click away to send your contribution to any software. But the problem is that any worthy contribution isn't an effort of a single click.
Doing any proper and useful contribution to a software is never done right the first time. There's a dance you will have to play. A slowly rhythmed back and forth between you and the software maintainer or team. You'll have to dance it until your contribution is correct and can be merged.
But as a software maintainer, not everybody is going to follow you on this choregraphy, and you'll end up with pull-request you'll never get finished unless you wrap things up yourself. So the gain in pull-requests here, isn't really bigger than a good bug report in most cases.
This is where the social argument of Github isn't anymore. As soon as you're talking about projects bigger than a color theme for your favorite text editor, this feature is overrated.
After working on OpenStack for the last year, I'm completely spoiled by our workflow and how it enables developer productivity. Recently I went back to just using git without gerrit to try to work on a 4 person side project, and it literally felt like developing in a thick sea of tar.
A system like Gerrit, and pre-merge interactive reviews, lets you build project culture quickly (it's possible to do it other ways, but I've seen gerrit really facilitate it). The onus is on the contributors to get it right before it's merged, and they get the feedback to get a patch done the right way. Coherent project culture is one of the biggest factors in attaining project velocity, as then everyone is working towards the same goals, with the same standards.