I think the main benefit with system mechanic is seen more with mechanical drives. I tested the software on a mechanical drive based system and noticed a slight performance increase, mostly just in the start time of the operating system. Besides that, I didn't really notice anything else.
I honestly didn't trust it enough to try and install it on my SSD based system. Mostly because I couldn't find any information describing what it actually did to "boost" performance.
My system(s) consists of keeping good file structures, watching what I install, and ccleaner. So I ultimately had no need to continue with system mechanic. Luckily I got my copy for only a few bucks so I don't feel so bad about throwing it out =).