I have the Calmini exhaust installed on my S/C Xterra. The install wasn't exactly "no hassle" I had to get handy with the torch to bend the rear section slightly to keep it from making contact with the spare tire. As for the quality, the tubing is ok, but im not too fond of the slip tube fit up on the muffler it will leak slightly, and as for the quality of the muffler its a piece of stamped steel junk in my opinion. I replaced it with a Magnaflow within the first month that happened to be the same dimensions and sounds way better.
Overall if you have the Non S/C Xterra I'd Recommend the Nismo I don't think they make exhaust for the S/C currently, the quality is top notch, I think it even comes with a larger Y pipe (calmini bolts on after the Y pipe) But the price is also high. I guess you get what you pay for. As for the sound of the calmini for the short time I had it on it was slightly louder and deeper than stock without the "rice" sound.