I know that DL4 has many haters (and lovers) for its delay modes, but for sure its looper its the best I have ever used, and I have used or checked out most of the loop pedals out there like Akai Headrush, Boss RC-2 (and RC-20), Digitech Jamman.
The thing I love about DL4 is that you can very easily do the things you want just by using one button at a time. For sure its the easiest and fastest of the above mentioned when it comes to using it live. Of course you cannot store anything like RC-2 so in that field they just cannot compare.
You can start overdub when recording or start playing the loop using seperate buttons, and also very importantly and as illustrated in the nice video below it is very easy to create one-shot loops and trigger at any time you want. Akai doesnt do that and the others need the loop to be saved first.
The bad thing about DL4 though is that its is very big and heavy so its kind of pointless to just use it as a looper, thats why I am currently using Akai, but I really miss its looper. It would be so cool (and possibly good marketing move) for Line6 to release some little version of that looper with basic functions, not like the big looper they introduced last year.(which i havent checked by the way)
check out Kele sampling his own voice.quite cool.
as you'll see the quality of the looped signal is close to perfect
[youtube][/youtube]
I've been playing about with the somewhat basic looper functions of my Digitech PDS-1002, and it's seriously making me want to get a full-on looper. It's meant to do delay, but it's a good advertisement for looper pedals indeed.
I'd like to go on record here and state that nothing has me reaching for my revolver quicker than a guitar line looped live. Seriously, every two-bit band is doing this now and it's getting old. Even overuse of delay is endemic round are way.