I always found Crucible much nicer and it integrates w/ Stash decently, but I worry it and Fisheye are on the backburner at Atlassian.
(Atlassian CEO here)
* Fisheye has crazy advanced searching (in commit messages, users, etc)
* Fisheye offers he ability to watch for changes on files and directories that happen in any branch
* Fisheye shows a file as it might look with all branches and tags together (though it doesn't always combine them well of course as might be expected w/out using the VCS's conflict resolution)
* Crucible shows you what changed since you last reviewed and has an awesome slider (though can be a bit buggy)
* Crucible live-notifies updates in the browser on changes
* Crucible offers reviews on patches to a repo that may not be committed
* Crucible differentiates comments and defects
* Crucible lets me see unread vs read comments on files
* Crucible lets me see the number of comments (read and unread) on in the file-list pane on the left before going to the file
I personally think all 3 tools serve different purposes. I understand the want to move away from Crucible/Fisheye which are older and predate Git which causes some issues. IMO, there is a big unsatisfied market for code review tooling right now. There is also a market (probably not as big) for intelligent searching and notifications across a repo.
Edit: Formatting
Re: stash. It's by far the best code review product I've used. And I've tried plenty!
In stash, I see the list of files on the left, and clicking on each file shows me the diffs. I also have the option to see the diff only (without the whole file).
Where I work we cannot use outside review tools, it has to be hosted internally. But I don't think we are your target audience in this case.
I figured that Reviewable wasn't the right tool for you / your company -- I was just looking for your opinion. Cheers!