I now have a One. It and the Quantum have been my main synths lately, as most of my energy has been going into sound design on them. The thing that bugged me on all the demos is that they immediately use one of the more unique features: the blend between the saw/tri and pulse core oscillators. Unfortunately, unless you're careful, it ends up just sounding like doubled octaves, and so every patch gets this harpsichord/organ vibe in which it's tough to tell the difference between the fundamental and the first harmonic. Finally I heard some of Jamie Morden's patches on it, as well as Mr. Firechild's piece with it, and I realized it had a lot more range than the default patches gave it credit for. (Jamie was contracted by Moog to produce a set of classic sounds which are available for registered owners free now.) So that convinced me to pull the trigger.
In some ways it reminds me a lot of the Prophet 12, but with better sounding oscillators for the traditional waves. The voicing is intentionally thin in some areas to get it to sit in a mix better than the Memorymoog did (so they claim, I've never had a MemoryMoog). But with three oscillators, the effects, and all the modulation sources, it's easy to thicken it up. So you can do traditional Moog (or Oberheim or Polysix or...) sounds, including a nice ring mod and sync controls. FM, too, but it's harder to control. It reminds me the most of the Prophet 12 in that there are so many performance options as well as neat tricks you can do--the X/Y pressure pad does a lot of what the two ribbons on the 12 do. The two filters take some work to get to know, but because the multimode 12db filter is actually also a dual filter with separable peaks and configurable in parallel or in series, you can actually get triple-filter sounds. That makes a lot of the standard filter sweep sounds a lot more interesting and subtle. Because the voice is complex, I rarely ever make layered sounds (this was true on the 12 as well).
It is more chameleonlike than the River or Deckard's Dream, but there are some definite Moog One-ish sounds, often involving the dual filter and mixed oscillators. But it can also double as a generic analog poly with some funky extras, as well. It does pair well with the Quantum, and I think it would with the PolyEvolver, because it has a complimentary character.
The damn thing will strain your back, though.