My first boot time defrag created "serious error" per MS error report.. Defragged the few system files then got blue screen of death (actually BS of Bad Injury :-) ) Restart hung on desktop with jerky mouse that wouldn't do anything. Next restart hung at Welcome screen. Same for nest restart. Finally booted successfully to last good configuration (running XP, SP3.)
This kind of stuff make users scared to death of these types of programs. I like the regular defraggler and with fingers crossed will use it again. But never for the boot defrag.