I am not a big fan of the sound of inline 4's but as they go, the N1K, sounds decent. I do like the sound of a high revving V-twin, triples and inline 6's and some V-4s can sound pretty cool as well. I also have to give Honda credit on their 270 degree parallel twin in the CTX 700 that when you get rid of the stock exhaust, it actually sounds pretty cool.
On the downside of this motor is it vibrates more than any other bike I've owned in recent memory other than a v-twin I have and it has to be spinning at least 10k before it has as many vibes as the N1K does. I am not saying it is terrible but at certain RPMs it is intolerable for me when riding more than about 15 min. I general the motor is pretty smooth so it is paradoxically in that sense; smooth yet vibey. Most of my other bikes the vibes are more linear in that at low RPM less transitioning to more.
On my N1K there appears to be resonant frequencies where it is like one big tuning fork at those RPMs and then goes back to being smooth for a while until you hit about 7K and from there on up to redline it progressively gets buzzier. Since I don't need to rev up there often, I don't have to deal with it and it works out to about 110 mph in 6th so to stay there long means likely going to jail unless on the track.
Still, niggles and all, the Ninja is a very good bike. It is an incredible value for what you get.