I just received one of these last week (SN16!). It's pretty awesome. I have an SF-2, a Series II equipped, Starfire, and decades ago, I built a Craig Anderton Super Tone Control.
This is very similar to a single channel SF-2 with the addition of the notch filter, although it feels like it has a wider variation in Q and can get much tighter. You can easily put it in bandpass mode and make it so that it only boosts one note. In comparison to my onboard Series II filters, it's much, much quieter and has a greater range. Of course, my SII electronics are from the early 70s, so they aren't state of the current art, although they still sound great.
The Anderton STC is based on the same idea, a state variable filter, but is quite a different beast. I put mine together ca. 1983 or 84 and decided to build it into my bass at the time. Instead of having a single filter output and then being able to blend it with the direct tone, it has three outputs, for below the filter, filter output, and above the filter, essentially making it a three band eq and a bandpass or notch filter. However, by setting the frequency and Q, you can get to the same place as a high pass or band pass filter. The filter range is insanely wide and the you can crank up the Q so high that the filter self-oscillates. Pretty cool stuff, but much less wieldy while trying to play the bass live. Ultimately, I took it out.
I think the Philter is a great box, although it does have a pretty steep learning curve. I've used it not only with my modded Starfire, but also with a Jazz and P bass and it is a fantastic addition to both. I've also been trying out the new Grace Design M303 DI box and the two could be a rig on their own with a power amp. I've played tons of DI boxes and at first, I though it was like most other DIs, perhaps a bit cleaner, but when going to other DI options, it sounds like a blanket is thrown over my tone. The low end is also really deep and clear. It's a great combination, given how deep and dynamic the Philter can be.
Alembic also came out with their version that has another setting, which is called bandpass+. It's a bit more expensive, but has some other features, including BP+
Here's the frame of 35mm film I got when I ordered the GD movie when it came out on DVD. The filter's in there somewhere!