I nearly gave this only 4 stars, kinda wanting to give it maybe 4.5, just not quite 5. But maybe 5, with a "but..." is close enough...
It seems to be the most reliable and fastest "dupes to clones" app I've been able to find. I've so far only run it on data I could afford to lose in a pinch if it screwed things up -- mostly old archives of stuff I probably don't really need but just don't want to throw away just in case. Haven't found any problems yet, although I've only opened (to test) a tiny fraction of what it's processed. Still, so far so good.
Also, from what I can tell -- well, at least, assuming what it's reporting to me is accurate -- it compares files a number of "shallow" ways (filename, size, etc.) and disqualifies anything that doesn't exact match through those methods, then hashes anything that might still be a match, to make a final decision. (Developer/author, please correct me if I've got this wrong at all).
Assuming that's what it does, then the end result should be (and so far seems to be) completely transparent (ie. before == after) except for a bunch of newly saved disk space.
In short, so far, it seems to do exactly what it says on the tin, and has proven to be the (almost) perfect solution to a particular problem I needed solved.
All that said, I feel like it really needs just a little more functionality and UI.
Specifically it's kind of all-or-nothing. Well... it lets me choose a folder rather than a whole drive, so some selectivity there. But a little more flexibiltiy would be great also.
Specifically, something like this: it presents in the log a very clear to understand list of all the dupes it's found. If it could present that list in the UI, each row with a checkbox to say "ok yes, replace this file with a clone" vs "no, leave this one alone, don't replace it with any clone, don't touch it in any way".
Then again, this might not even be the right solution to the problem I'm really trying to address, that is: just hitting a "Deduplicate" button and letting it do its thing requires an awful lot of trust, especially on potentially irreplaceable data. Not security or privacy type trust, just trust that it really is doing what it says its doing and there isn't some bug that might destroy some of my data. Perhaps, for example, is the "no collision" thing absolutely guaranteed?
I guess I'm not really sure what the best solution to that is. I put a lot of the same kind of trust in my backup apps, but then again I back up my data three different ways with three different apps, so if one of them destroys my data or fails in some way I've got the others. In some sense that applies here also, so maybe that's good enough. But I think the above extra UI (the list with ability to uncheck some items) would still be a really good option to add.