I used a boss RC-50 for a while, had all my synths plugged into a mixer, and the mixer's stereo out into my audio interface. It worked pretty well, here's a song I wrote while jamming with it:




After a while I got tired of the mediocre converters on the RC-50 and the fact that I couldn't tweak any sound after looping it - so I decided to switch from audio looping to MIDI looping and bought an MPC 2500 - which is much, much more convenient when working with synths and drum machines. Now I can easily loop anything and then tweak and change the sound to my heart's content, record on seperate channels on my DAW etc.