Firstly, I will prefix this review by saying that I didn't buy this synth in order to get the Jupiter-8 and Juno-106 plugouts as they're not really my cup of tea, however, they are very well implemented on the System-8 and sound fantastic.
I just use the System-8's native engine as I believe it sounds really lovely. It is very versatile sonically and creating sounds with a knob-per-function interface is a true joy. There are lots of filter types (18 at time of writing) and lots of oscillator shapes - including one called "FM" which is very handy.
Many people complain that it only has one LFO but this isn't a limitation at all because like the Korg Minilogue, it has a 4-lane motion sequencer - except on the System-8 it has 64 steps so you can "record" all manner of movements into your sounds and play back those movements for every key stroke. This allows you to completely shape a sound to your liking in an intuitive way by "playing" the controls as you record them as opposed to having to "program" those changes with a complex modulation matrix etc.
The other benefit is that it can output audio via USB at 96kHz along with the MIDI so that saves a channel of your audio interface as well!
All in all, it's a fantastic synth. The only downside to some is that there is no aftertouch on the keyboard, but that doesn't personally bother me.
I would definitely buy this synth again if anything happened to mine. Highly recommended.