Basically the reviews on System Mechanic are mediocre
I've been a System Mechanic Pro user for ~10 years, and it's the ONLY paid utility I use. I'm not sure which reviews you're looking at but it's rated highly by editors at both PC Magazine and CNET (see below). Cost is actually $19.95 per year for unlimited licenses per family after easily accessible discounts. It has done an especially good job of keeping my 100 year old father-in-law's PC running in spite of the abuse he gives it. He lives 60 miles away and SMPro has saved me LOTS of time and travel to fix it. I have never found it necessary to contact SMPro support for any problems but I'm not surprised they charge for that.
PC Magazine Editors rating: Excellent
If your PC isn't blazing along at the same pace it did when you first pressed the power button a few weeks, months, or years ago, you should invest in Iolo System Mechanic 14 ($39.95). The tune-up utility suite whips your computer back into shape by defragging the hard drive, repairing Windows's problematic registry, and tweaking CPU and RAM usage in real time. With this iteration, Iolo System Mechanic includes an unexpected and welcome new feature, PowerSense, which detects your PC's activity and throttles the CPU up or down as needed. Iolo System Mechanic 14 is pricier than some of its competition (like Slimware Utilities SlimCleaner, the PCMag Editors' Choice for free tune-up utilities), but Iolo's utility comes packed with features that make it worth the price of admission. It's our Editors' Choice for paid tune-up software.
CNET Editors rating: Outstanding
download.cnet.com (don't worry...a review page and not a download)