High Frequency Stimulator

This one is definitely worth having in your plugin folder, although, personally I rarely use it much. I should qualify that by stating that it is purely designed as a harmonic enhancer with no other gimmicks up its sleeve. It's not one of those mysterious enhancers that use a mixture of band-split delays, harmonics, compression, Eq and phase correction. So you can't compare this to a BBE or Aphex etc. You could use it as one of several insert FX in a chain to achieve that overall enhancement effect though.

GUI - it's obvious and simple - nothing flashy, but for what it does it would be a waste to have CPU taken up by flashy graphics.

Features - minimal but useful - a frequency scale which allows you to control which frequencies and above it enhances, and a dry/wet mix basically. Doesn't need anything else. I would rather it come in a choice of VST and DX - but that's only personal preference - it works very well as DX only.

Stability - very stable, very little CPU and I've never had a crash or conflict with it.

Manual/Presets - doesn't really need any.

Sound - It has definite uses. I find it good for drum tracks/groups, to add a little sparkle, but with the proviso that you should be careful about using it on hats and cymbals - it tends to make them a little too harsh and grating. That is no problem with HFS though - all harmonic enhancers do that to hats if you don't use it very subtly. It works well on snares, toms and other drums though.
I find it works slightly less well on instrumentation unless used very very sparingly, but again that is down to the nature of the process rather than the plugin itself. If you use it 100% wet and then solo it you hear what it does to the sound, and it seems to add the harmonics in a true manner, with no Eqing tricks or deceptions going on.

I would say it's safer to use this kind of technique on group tracks rather than over a whole mix, although you could do that also. It's especially useful if you use alot of drum samples (or any kind of sample) say, off sample CDs - many of which are often taken off vinyl, so they tend to be a bit lacking in top end. It's generally a safer bet than using too much Eq which can be dodgy unless you get it spot on, and use a very good Eq.

Really this is almost a one-off. The only other competition I'm aware of is the DSPFX harmonic enhancer, which is not free, but offers the choice of odd or even harmonics (personally I find no use for even harmonics though). So it stands out as filling in its own niche. If you had good comps, Eq and band splitters, then you can certainly use this in your mastering chain, so it can be versatile.

RGC do tend to make different and unique s/w and this is no exception - it's good to have it for free, but you probably won't use it an awful lot. But a thumbs up for a useful plugin not covered by other companies. I'd rather have this than some of the plethora of same-old plugins in other areas provided by other companies.