The Boss EV-10 is a simple expression pedal for use with Boss effects and other pedals. The specific pedal that I've used with EV-10 with has primarily been with the Boss PS-3, which is a pitch shifting and delay pedal. I've also used it a good amount with the Line 6 DL4, as it's compatible with this line of pedals a well. There are a number of Boss pedals out there that have an expression pedal option, but the PS-3 is one that needs it the most since it's a pitch shifting pedal and it's nice to have control over this while playing (something not possible without the expression pedal). The EV-10 is suitable with other brands of pedals as well, but the studio I work at has it for controlling the PS-3. It's really a simple foot pedal, with just a single 1/4" jack for plugging this into whatever pedal you're going to use it with, and a knob on top for setting the amount you'd like going out of it. They don't make the Boss EV-10 expression pedal anymore, but you can still find them used now and again on Ebay and elsewhere online. There's nothing really special about the pedal - it does what it does and that's it. However, what it does can certainly be helpful to you if you have a pedal that has an option for one. If you don't come across this pedal at a super cheap price, I'd recommend going with the newer Roland expression pedal which is cheap enough new anyway. It's probably worth it to even spend a little extra on the new one as to be sure that it doesn't have too many miles on it. So while the Boss EV-10 is more of a necessity than a luxury in some situations, it's one that does it's job well even if it's on the older side...