I'm not familiar or haven't try Registry Mechanic but I'm using RegSupreme Pro by macecraft.com and Microsoft's Live Safety (online service) that also includes registry cleaning.
There's no need to use 2 registry cleaners but I like to use them both. If I let them scan the system, both are finding similar items but the #s is not exactly the same. Difference of 5, 10, 15 items for cleaning only.
I have no problem so far. Note that I first run RegSupreme Pro since it's designed for it instead of "additional service/feature only" and it has built-in back-up tool before it will fix the item, whereas Live Safety requires System Restore to undo changes.
Let's hope to hear some words from Registry Mechanic users too.
Something just dawned on me while I was cleaning up my computer this morning. I have Windows Live....and Registry Mechanic. Windows Live scans and cleans the registry and so does Registry Mech. Is there a problem using both? Does one mess up what the other has done?...Maggie