I have the Soundsmith Strain Gauge, got one used about a month ago. SG does not use RIAA or any kind of normal phono stage because it is not any kind of normal cartridge.
MM and MC both are generators that generate a voltage by moving. The faster they move the greater the voltage. SG does not work that way at all. What it does, the cantilever is connected to strain sensors that detect deflection. These are used to modulate a voltage coming in from the SG preamp. So the preamp generates the voltage, the cartridge modulates it (similar to the way a tube acts as a valve) and the resulting signal comes back into the preamp where it is amplified.
Because of the way this works it requires no RIAA equalization. One of its many advantages over the conventional approach.
Overall it is a nice step up from Koetsu Black Goldline/Herron VTPH2A. With one exception, the low end is a bit weak in comparison. In terms of tracking though it is darn near seamless. Recordings that seemed to have hopelessly large amounts of sibilance now sound like the normal level of sibilance you would expect from a real live person. Percussion in particular is tracked exceptionally well. Listening to SG confirms Koetsu has a real talent for tailoring resonance. The Black Goldline may be entry level Koetsu but it truly is a wonderful cartridge. The SG on the other hand gives the distinct impression of listening to the master tape.
SG also has some nice features like auto-mute and a built-in 12Hz filter. Its main weakness would seem to be the power supply, which can be upgraded with SBooster, Farad, etc.