This is the synth that brings good-sounding and easy-to-use additive synthesis to the masses. Or, at least to those parts of the masses with some $$$ to spare.
The single worst thing about Spectra is really the way it risks being overlooked in today's "louder and more-complex-than-thou" market. At its heart, it's a synth for the traditional composer/musician, rather than those who'd like to build entire pieces around a single complex, endlessly evolving, infinitely modulated synth patch.
Sure, Spectra can do complex: The oscillator section can morph seamlessly between different harmonies (waveforms) 99 times in the same patch if you need it to. And with two LFOs plus looping envelopes for pitch, amplitude and filter, you won't run out of options in a hurry.
It's just that when someone asks "Well, what can Spectra do that others can't?", the answer doesn't lie in a number of features that almost never get used but look good on a spec sheet: Instead, it's all about sound quality, CPU efficiency, versatility, and ease of use.
Every basic element in here is top-notch: The sound generation, the filter section, the effects (what else would you expect from a Kjaerhus product?), and not least the two-bank patch management system.
Since there is no limit to the waveforms Spectra's oscillators can produce (given the right input), and the number of "analog-style" filters included, Spectra can serve up anything from fat analog sounds to FM-like timbres -- although sticking with the internal harmonics editing tools, you're far more likely to initially end up in Yamaha DX territory than in Moogland. No worries, just use the wave analysis/import function, direct it at some fat waveforms, and voilà!
By the nature of additive synthesis, the one thing truly lacking from the sound palette is very breathy/windy/noisy types of sound -- despite the included noise generator. As of now, you can only change the noise colour and set the overall noise mix level for the patch. I would very much like to see some added noise shaping tools in coming versions.
Anyway, it's not much use going on and on about how it looks, sounds and works: Just download the demo and try it out. Once you get your head around the concept of additive oscillators, and discover the workings of the wave analysis/import tool, you'll surely be amazed at how fast you'll be making your own, great-sounding patches!