The Boss NS-2 is a noise suppressor / noise gate pedal - maybe not the most exciting purchase a guitarist can make, but definitely an important one if you are playing anything with high amounts of gain, hum, or noise.
The Boss NS-2 operates similarly to most other noise suppressor pedals on the market - you set a threshold so that only signal above that level will pass through to your amp. The pedal offers a Decay knob to set the speed and shape of the suppression. The final knob sets the mode of the unit, which changes it from a suppressor to simple mute / killswitch pedal. The NS-2 also features a built in loop, allowing you to patch in overdrive / distortion / fuzz / other noisy pedals directly into the NS-2.
The NS-2 has a fantastic set of features and sounds excellent. It effectively cuts out noise, giving the user full control of the amount of suppression, but it also properly retains the sound of your guitar.
I think the NS-2 is an excellent noise gate pedal, but it can be difficult to find the best place for it in your setup. The amount of features the NS-2 offers, and the position of it in your pedal chain can vary the results and success of the the pedal. It takes a little time to work out the best way of using it for your own personal use, but it is totally worth it!