I got one. Used it only once so far and can't really judge it since it was at a gig and didn't have an isolated room to test attenuation vs amp -> cab scenario.
Having said that, I love what I was hearing from my cab, so it's a winner for me. I just had it plugged in and it was at minimal attenuation setting. I have only had experience with the Marshall Power Brake, and that was no bueno for me. Total tone suckage and limited options. I bought the Power Brake for $20 and sold it for $200 so massive win for me
Only thing I don't like about the Jettenuator on paper is only 1 output per ohm. I'd love to have dual 8 ohm outs on it, but it isn't a deal breaker since I use dual cabs 0.000001% of the time
Best thing about it. LOAD BOX. Can record at night direct to my PC without connecting to a cab via Line Out.