OWLTECH  I have had TechTool Pro in some version ever since it came out. It has gotten me thru some really difficult problems, where other products did not even recognize the problem.

However, the later versions, beginning with v4, have proven unreliable to the point of even destructive. I remember trying to recover an HD that had a problem with double volume bitmap allocation, probably caused by prior application crashes. Running TTP rendered my flaky HD totally unbootable, and ultimately unrecoverable: I had to start over with reformat, a new OS, and scraps and scrapes from various backups. Since then, I have kicked the product to the ditch.  
(Version 4.6.2)