We use and love Atlassian Stash (https://www.atlassian.com/software/stash), it's excellent for code reviews. The diff view options and commenting make it easy to create pull requests, get feedback and eventually merge code into the master branch. (Per branch permissions are awesome too.)
Long time ago we also assessed GitLab and found it usable but not quite there yet for production use. Though it looks nicer now.
I use Stash and I'm quite satisfied with it. The ability to create branches from "inside" JIRA and then check out in the desktop from Stash is good for newcomers to Git.
Plus the ability to prevent pull requests from being merged without X approvals, at least one successful Bamboo build etc. It's all very well integrated and polished.
I'm just waiting on rebase instead of merge for pull requests.
I love the approval merging too, also the locking of branches (e.g. only X people have the ability to commit to master, everyone else has to have a PR reviewed and merged).
I wish it allowed comments in more places than just PRs, but it's been pretty great so far.
The only thing that puts me off Stash is that I'm a part time JIRA, Fisheye, Crucible admin (on site ~ 250 users) and my word both those things make me cry at least once a week with problems. If it's not dealing with the licensing hell and the tears of management when another $32k pisses down the drain on maintenance (mafia style protection) it's dealing with memory leaks, crashes and constant performance bottlenecks. And we run this on a cluster of three high end 8 Core/32Gb RAM/SAS disk HP machines...
I couldn't face Stash after that.
Edit: Atlassian support are excellent but we have to bug them too often with entire sites down (workflow consistency issues fucking the entire JIRA instance which happens at least once a month) for the product to be considered quality.
We've been using Stash as well and I'm, for the most part, quite pleased with it. My biggest initial problem was a lack of GitHub style, convenience feature. In the last couple updates, they've filled in the biggest feature gaps and demonstrated they know what they're customers want.
I am using Stash also. Love the new side-by-side diff. We switched from Gitlab to Stash few months ago. Mainly because Stash upgrade was way easier than Gitlab at that time.
Thanks for the feedback (and others) on stash. Right now we're down to choosing between Stash and a couple others, and it seems to be the best choice for the team.
Hey anything is better than SVN which is what we use now!
On the Admin side it has been easy to get up and running for testing, the upgrades are pretty painless and come on a time based release cycle. The docs are decent and there is a rest api and the company has been very responsive to tickets we file. And it is _significantly_ cheaper than github:fi.
On the user side it hits all of the big items and the interface is useable. It doesn't have some features like GitHub's image diff or gh_pages support, but for many things there are Stash plugins that either exists or you can write. And they provide an easy way to not leak internal url's while using gravatar (http://benjamin-meyer.blogspot.com/2014/03/how-to-stop-leaki...)
The biggest downside I have at this point would be that out of the box the permissions model is not as rich/robust as gitolite. Although most Git frontend servers are no where near as rich as gitolite.
Developers will love it. For what it's worth Stash fits perfectly in our review process where anyone can be assigned to review and comment on code. The pull request diff explorer makes it super easy to track which files have been reviewed already and contain comments. It really has increased our overall effiency and shared knowledge as a team of seniors and juniors.
Long time ago we also assessed GitLab and found it usable but not quite there yet for production use. Though it looks nicer now.