I'm still running a pre-release Super-PID, after many years.
As Louie said, it's hard to recommend them today, due to the low price of VFD+Spindles.

A 2.2Kw spindle and VFD is cheaper than a $200 router + Super-PID, and much more powerful.

But if you already have a good router, and only have 110V, than yes, the Super-PID is a worthwhile investment.

The SuperPID will fluctuate more than 150rpm's at the start of a cut, but only for a fraction of a second. When actually cutting, rpm is very stable.

And yes, it'll run much cooler, at all rpm's. The only issue you would have is if you're trying to do heavy cutting in the 5,000-8,000 rpm range.