Code Compare is a compare tool designed to compare and merge differing files and folders. Code Compare integrates with source control systems: TFS, SVN, Git, Mercurial, and Perforce. Code Compare is shipped both as a standalone file diff tool and a Visual Studio extension.
If your developer team is well-versed with standard VCS then I don't think they need this additional software to occupy their memory, they already have those capabilities within all VCS. A well-suited use case can be a team where not everybody has knowledge of VCS but has to collaborate on a project.
Upsource is the best review tool we've found but it still has some flaws. Notably, it makes reviewing small and quick changes less convenient than they need to be, and diff viewing (especially collaboratively) can be tedious.
It does handle larger, iterative reviews well. Especially when using a feature branch, Upsource will track that branch and automatically add all commits to the review. You can then review the branch as a whole, or look at a subset of diffs.
Creating and closing reviews isn't as quick as it could be. You must create a review, assign reviewers, approve and close. I wish there would be a quick review-approve-close for a commit where the change is simple and doesn't require multiple review iterations.
Web based interface can be clunky, especially when looking at big diffs side-by-side
JetBrains IDE integration is somehow less convenient than going using it in browser.
Compared to the other tools we evaluated, Upsource was the only tool that allowed distinct reviews without needing explicit pull requests while still being able to go in-depth when required. The diff viewer is serviceable and better than the alternatives, as well, especially the side-by-side viewer.