I just picked up one of these little guys and I am impressed.
It is obviously a very simple device.... it does 2 inputs with one output, or vice-versa.
Great for me as I use 2 different basses on gigs and it is much easier to keep both of them plugged in then muting my amp, unplugging, etc.
It would also work well for anyone (mostly guitards) that needs to quickly switch between 2 amps.
Not noisy at all, built like a tank and at $35 new, it is great value.
Not what I would call the most exciting "effect" out there, but it does what it is supposed to do.
http://www.bosscorp.co.jp/products/en/AB-2/