I picked this up in a bundle with Unison and Adder for $70 from ConcreteFX. I wasn't expecting to get much use out of this but I've been very pleasantly surprised to find that this is my favorite of all the additives out there.
First, I love ConcreteFX's unconventional GUIs. They're one of the few developers bold enough to abandon the hardware-style guis most developers favor. Their guis are more simpler and clearer and appear much more at home on a computer screen.
Second, this is easily the best *sounding* additive out there right now. I've been pretty disappointed with the new additive synths that have come out recently because, while they offer tremendous flexibility, they just tend to sound thin and hollow. Adder has a thick, rich sound that holds it own against a good subtractive synth.
Third, the modulation possibilities are mind-boggling. Adder includes a step sequencer, a HUGE mod matrix, fm modulation among all of the partials, a full-featured fx section, and a good randomizer. The gui is so straightforward that it isn't immediately clear how deep Adder can be, but the possibilities are endless.
Fourth, at a price around 1/3rd of the competition, Adder is scandalously cheap. It would be well worth the $200 or so other companies charge for their additives but at the current price it's a must (even more so with the current bundle that throws in Unison/Micron and Digital for no extra charge).
Documentation is somewhat sparse but adequate and the gui is very intuitive for a synth of this complexity.
Adder comes with a somewhat smaller preset library than the other additives but there are a lot of very commanding presets in the library.
ConcreteFX synths are highly underrated and the unconventional guis and low prices probably make people take them lightly. Don't make that mistake.