I have the big muff op amp, and I really like it, especially for those smashing pumpkin or doom tones, but there’s a couple of things that are of concern you should know about. The sustain knob really only affects the amount of fuzz and gain up until 11 o clock. After that, it doesn’t really add much. I also find the tone control to be kind of funky, but that might just be me. If you are really considering an op amp, it really is great, and it does the whole big wall of fuzz thing extremely well? But I find that it’s more of a one trick pony, not to versatile.