I have a Boss NS-2.
I feel the Boss is great when it's in front of the amp, where I swear by it, but I feel in the loop it doesn't work very well (Toe Knee has described the problems in the loop. If your amp is noisy, then it's not a good choice. In my case, my amp is rather quiet and the Boss works great to remove the noise from pedals and pickups, even single coil hum. To be fair to it, the NS-2 is not designed for the effects loop in the first place (according to Guitarist).