Endless Series is a unique high-quality sound mangler and generator, sound designers and film music producers should recognize the usefulness of this effect immediately.
Endless can make extremely cool and psychedelic flanging/phasing effects which are like nothing else on earth, it is fascinating to play with and listen to. I've found the effect works well on vocals, drums, synths, and especially guitars.
I haven't used the synth part directly, but it is helpful to familiarize one with the effect, and not only is investigating the psychoacoustic shepard tone effect educative and eye opening, but it's also fun!
You wouldn't know it from the plain looking UI, but it works in a very sophisticated way and feels extremely refined with many functional touches to help the user to control the effect. It can be used subtly as a treatment, or it can create it's own drama. This wide range and sophisctication of control makes using Endless a pleasure; so although it is unique, it is easy to discover potential uses for it and to imagine more.
As a plug-in developer myself, i know oli larkin is competent and capable to support his products; Endless is likely to be around in the future, and be maintained and improved. So I think Endless Series is a good buy, like a cool hardware stompbox. imo, if you like flanging/phasing type effects, or other worldly sound effects, you'll want to own Endless Series. And I think it is noteworthy that there is no other plug-in or hardware device like it.