I used Amnesia for the full trial period, and for the last week compared it directly to CleanApp. I've also tried a few other uninstallers over the past year or so. While I like Amnesia's interface and design, functionally it falls short of these other uninstallers, particularly CleanApp.
Note that my comparisons are made using CleanApp without the logging agent installed. I prefer to keep as few background processes running as possible, and IMO a logging agent is too much overhead for an uninstaller app.
On my Powermac G4 FW800 system, Amnesia is significantly slower than CleanApp, churning the HD for several seconds after dropping an app on its window before listing associated files, compared to CleanApp's nearly instantaneous listing. Surprisingly, the files that each uninstaller identifies are not always the same; out of perhaps 20 "uninstallation," CleanApp found extra files in two instances. The other side of this coin is that CleanApp tends to list false positives, files unrelated to an application, while Amnesia never does. I prefer getting false positive hits, but for users less familiar with Mac OS X, Amnesia is the clear choice. Blindly accepting all of CleanApp's hits for deletion would soon cripple other applications.
My main issue with Amnesia are its quirks. If an app's purpose is to delete files, then it should never flake-out, yet Amnesia did this several times over it's trial period. Twice I dropped apps on Amnesia only to have Amnesia endlessly thrash my HD until I force-quit it. After a system reboot, dropping the same apps on Amnesia worked fine. And once after clicking "uninstall," Amnesia began churning and thrashing for several minutes. I checked the trash to see if Amnesia had deleted anything, and discovered that it had moved 2/3rds of my "Applications" folder to the trash. I suppose this could have been user error, but shouldn't an app designed to uninstall individual applications and their associated files give a warning of some sort before trashing the entire "Applications" folder?
Ultimately, I see little need for uninstallers like Amnesia. 90% of the time, the files left after dragging an app to the trash are harmless. For the other 10%, the files are always in a handful of locations where they are easily found. I also use the freeware app "Find Any File" which will search the entire HD. Just run a search with the app's name, and the developer or software company name.
If I'm really serious about removing an application entirely, I use "Uninstaller". For example, I tried out a humongous set of HP scanner drivers, and wanted to be able to remove them if they didn't work. Uninstaller scans the entire HD and saves its state, after which you install the software, then Uninstaller scans the entire HD again and reports exactly what files were installed. This is a brute strength approach but it is 100% effective, and the only approach worth paying for IMO.