Not specific to this app, but I've never seen a computer cleanup app that I would recommend to anyone but a very knowledgeable person (like you, Dave).
All the ones I've tried have flagged many files I knew to be necessary for some app or other. And all have flagged duplicates that I could see on inspection were not identical. (I just saw a mention of a duplicate remover that used a more sophisticated system to identify duplicates, maybe on Macintouch. See below.) They make a list of files (often dozens or hundreds) and you have to determine which to really delete.
This includes Duplicate Annihilator for iPhoto. I used it a while back because I'd accidentally imported a bunch of duplicates that were scattered by date. I had to inspect every flagged photo. It was mostly right, but not mostly enough.
Uninstallers made for a specific app should be safe.
It's rare that a leftover file can actually cause problems. In most cases, I wouldn't worry about leftover files, unless they're really large, like runaway log files. The myth that leftover "cruft" makes Macs slow is just that, a myth. (That myth may arise from the industry in "cleaning" Windows computers.) There was an instance of an Adobe Updater for CS5 that failed because a particular file was missing, usually removed by a cleanup app. (A very old X-code installer is one recent example of a dangerous leftover. The new installer, combined with the leftover, caused data loss for a few folks.)
For many Macs if your hard drive is getting too full, it's time for a new drive. If it's a recent iMac that's hard to open, a new external drive is a temporary answer, though personally, I might take my iMac to a shop and get a new, much bigger drive installed. (The recent iMacs require the front glass to be removed, then the whole LCD monitor to be removed to access the hard drive. What was Apple thinking?)
So, bottom line, I'd suggest you test this app thoroughly on a test Mac before using it yourself on a client's Mac or recommending it to SMUG or a client.
PixelEspresso's Decloner 1.4
is a utility to find and delete duplicate files. It uses a secure hash algorithm (SHA-1) to determine if files are identical, presents the list of duplicate files, and supports both individual and batch deletion. This release adds multiple folder support, display of files that couldn't be removed (with an option for permanent deletion), an option to replace deleted files with aliases of the remaining file, accessibility improvements, and other changes. Decloner is $19.95 for Mac OS X 10.6 and is also available in the Mac App Store.