It's hard to tell if registry defraggers and cleaners work, but the Auslogics Registry Defragger looks good. It's hard to gauge if these programs are effective because once you're done using them, you'd need more than a mere store-bought machine with store-loaded programs to judge CPU speed.
When you run it, it tells you that it will perform a registry analysis, after which you will be able to review its registry report, and once that's done with you can run the registry optimization, requiring a reboot to defrag and compact the registry. It's a fairly serious program, and once you begin the analysis it won't let you move the mouse outside the program window. It also "strongly recommends" that users close all other programs while it runs, although this is a standard warning for registry-cleaning apps.
An expected change in clock speed or other performance benchmarks never materialized. Resource-intensive Internet browsers and e-mail clients may be piggish, but they're necessary and don't load any faster or slower than normal after running the reg defrag. Until a registry defragger or cleaner can demonstrate benchmarkable improvements to a computer, they'll be little more than 21st century snake oil.