I just installed the Side Trail exhaust on my X3. The sound is a nice deep tone and easy on my old ears. 30+% quieter than the removed HMF exhaust. The Treal exhaust is very high quality. I have the EVO 3R tune. I noticed the car took a minute to adjust when I first started it. I had some back firing or low rpm bogging and then after a few minutes the car adjusted and ran normal. I did use some copper anti seize (sensor safe) on the O2 Sensor.
I experienced a problem while mounting the rear muffler due to me keeping the HMF frame support. The front holes fought me a little because the HMF mounts holes need to be ovaled. The V clamps come with normal non locking nuts. I grabbed some Fuji locking nuts to double nut the V clamp.
I would definitely buy this exhaust again and recommend it to someone that wants high quality, dampened sound while keeping the higher flow.