This is a Jack Dark Nintendo "emulation." I am not a hardcore chiptuner, so I do not know how closely he's emulated the capabilities of the NES hardware, but from some of the sounds on tap this seems pretty authentic. The built-in methods of modulation usually sound very much like the tricks I heard on a lot of old NES games in my youth.
I like this better than most of his older synths. But like those synths the behavior of Syntendo can be a bit unpredictable and unstable in various hosts. You'll find knobs and functions that don't consistently respond to tweaking in any host, although I've not yet had Syntendo crash ASIO or the host itself as many of his older plugs can do for no apparent reason. The GUI is not terribly attractive and is also confusing, particularly the step sequencer. Mouseover knob tips are what you get to figure things out and even then they're not terribly helpful. Plan to spend more time than you would on average figuring out how to make this thing create useable sounds.
The step sequencer is particularly messy and problematic. It only works in octaves, but from the pulldown numbering you'd think it would refer to diatonic scale steps. And it's actually mostly useless because it throws in random "baseline" pitches when it's supposed to be silent.
Like most J. Dark plugins this comes with a small and hilariously named set of presets that are certainly way out there but not terrifically musically useful. But spend a little time tweaking this and you can get REALLY ballsy sounds out of it-- it is admittedly a really killer bass and lead synth if you are willing to spend the time.
Compared to things like Magical 8-Bit and especially Tweakbench Trilogy, this is better for authenticity and much more flexible. To get better tones for fake chiptunes you'd probably be looking at plunking down some cash for reFX's QuadraSID. And of course, it's free, and I for one am very thankful. Just don't expect it to be the most stable or predictable VSTi you've ever used, not that this is usually what Mr. Dark strives for. Flaws and all, it's really one of the cooler "niche" VSTis I've used in a while.