Note: I'm using Syncla-X in VSTHost and Cakewalk on Windows 10 (64-bit). As I don't own a Synclavier nor do I ownSynclavier V, I can't compare the sound.
Syncla-X is a free emulation of the NED Synclavier II's FM/Additive synthesizer component (i.e. no sampling/sequencing).
Each of the additive oscillators produces eight sine waves tuned to the first eight harmonics with controllable amplitude for each harmonic (partial).
The oscillators are then grouped into four carrier/modulator pairs and each given their own ADSR envelope (so. essentially. four layered two-operator FM synthesizers with additive oscillators). You also have control over the carriers' frequency and volume, Fm, Fm Fine, and Impact for each carrier (not sure what these do) and the panning of each voice (two-operator FM synthesizer).
The signal then passes through a global ADSR envelope and a resonant low-pass filter with its own ADSR envelope. Global tuning, volume, velocity sensitivity, bit depth, tremolo (from the main LFO), FM Mod, FM LFO, (not sure what these do either) are all available.
The UI is very good looking and easy to use. I would personally prefer Helm-style visual envelope depictions, but that would probably be difficult to implement, and I can't really complain about that, seeing as very few software synthesizers (free or paid) have that capability.
The only flaws that I've noticed are that the markings that show the knobs' position tend to blend in with the rest of the knob (the developer is aware of that and plans to fix it), and while it just fits on my screen (I'm fine with that, I like large interfaces), it's probably too large for some monitors.
Note: I'm new to FM synthesis, and am learning as I go. Therefore, my sound creation abilities are somewhat limited.
It sounds very much like how I imagine a very early digital synthesizer to sound, in a good way. It's capable of very nice organ, bell, and pad sounds. I can't really compare it to anything I've used, but it sounds good. Really good.
The seventh and fifth harmonics might be out of tune (only slightly for the fifth, very noticeable for the seventh). I'm not sure if this is normal for a harmonic series or not, but it can be annoying.
Ease of use/sound creation
Syncla-X is quite easy to create sounds with. Two-operator FM is (for me) much easier to understand than the 4-6 operator FM used by most other FM synthesizers, and Syncla-X's layering and additive capabilities make it very powerful for a two-operator FM synth.
It's a free emulation of an absurdly expensive synthesizer, sounds great, and is fun to use. I'm giving it 4/5 stars for the small flaws listed above (I'd give it 4.5/5 if I could). I highly recommend Syncla-X. It's a great product, easily my favorite 2018 Developer Challenge entry, and (in my opinion) one of the best freeware synthesizers.
Does not work for me at all in several hosts I tried... 'Have you installed VC runtimes c++ 2015' -> Nope. I wanted to, but its installer says that a newer version is already installed. VC runtimes c++ 2017! Did some research on the Internet and there it says that VC runtimes c++ 2017 should be binary compatible with c++ 2015... I'm running Windows 7 64-bit with all Windows updates installed. My hosts are all 32-bit versions (Bitwig Studio 2.4 Demo, Tracktion 6, Ableton Live 9.7.5 Lite, Cantabile Lite 3, EnergyXT 3.0, SAVIHost) and I tried loading/using the 32-bit version of Syncla-X VST 2. Looking forward for a version that works for me.