I have used it since it first appeared last year. I can't honestly say if it has made any particular difference because my PC is fast enough for me, and I take good care to make sure I don't get hit with nasties of any sort. I like the one-click maintenance, it is quick and I guess it does its job very well. All I can say really, is that I am continually amazed at how many people have so many problems time and time again. I dare to say it - I don't have problems, other than little niggles usually of my own making!
I also use AVG/Sunbelt Pers Firewall/Ad-Aware/CCleaner/AntiSpyware Bot/RegClean(Paid for version) and Diskeeper.
Plus I think my fair share of ordinary Common Sense!