Firstly a brief description: the DD-20 is a digital delay with 10 different modes + 1 looper/stacker. The maximum delay time length is 23 seconds (enough time for many loopers including me) in almost all modes including the looper mode. These modes are: reverse (very pleasant I think), modulate, smooth, pan, dual, standard, analog, tape, warp, twist and finaly the loop mode: SOS. There are two type of presets access, manual and memory (four presets wich can be personaly change). The box has two pedals, the left for on/off the effect and the right to swap between manual or memory (I 'll explain later the advantage of that).
Well, the first thing that blew me is that the DD-20 is a TRUE STEREO digital delay even in the SOS loop mode. This means if you play like me a stereo instrument (the stick) or modern synth devices, the L/R path are maintained (input and output). Very useful to play solos on looped chords and basses. The second thing is that in fact you can have two loops with the same length or not (maximum length between the two 23 seconds, for example 16 sec. + 7 sec.): play the SOS loop on manual mode and the second loop with one of the ten others mode preseted in one of the four memory banks with a long feedback and delay time. Switch between the two loops with the right pedal and you'll discover a marvelous world of posibilities. The only bad thing is when you switch back to the previous loop the stack is deleted. So you can play with that too.
Finally, this device is cheaper than his "big brother" Boss RC-20 or the Boomerang or the Line6 DL-4 wich are all mono devices in loop mode!
Sorry for this approximative english!