CRC - the VFX SD is a great digital synthesizer. The presets are hilariously late 80's sounding and barely hint at what the machine if capable of, but it's a very deep synth. The voice architecture is pretty unique - a "sound" can be made up of six "voices", where each "voice" contains one oscillator, one LFO, 3 (loopable) envelopes, two (non-resonant) filters and 15 modulators. You can then route the voices through two effects (that sound great in a semi-trashy way). So far I typically use two or three voices in a sound, but if you want to make something like a pad with six detuned saw waves, all going through their own filters with their own LFOs, you can do that.
Took a bit of time to get to grips with how it's all organized, but when it clicks it's really easy to program in a way that's typical of earlier Ensoniq stuff. Those guys sure knew how to make an interface for a digital synth. Too bad they sucked at making power supplies (see below).
Soundwise it's cleaner than an ESQ-1 or SQ-80, but it's still pretty grungy for a digital synth. It's not the machine to buy if you want something that will sound like an analog synth. It's unashamedly digital. It's great for pads and glassy/metallic weird sounds that digital synths excel at. And hey if you want to layer that kind of strangeness with some badass marimbas or slap basses, you'll be in heaven!
There are known and well documented issues with them (a keyboard calibration issue with a simple fix, and power supply heat-related issues that can cause it to crash) so it's a bit of a risk, but I knew this one had the keyboard fix already done. It has crashed on me a few times already, but that was after having it on for hours on a hot day. Pointing a fan at the heatsink seems to help.