My very quick review of loopers I've used for guitar.
Akai Headrush E2 - Short loop times, real cool non loop mode that does multi head tape out, two switches, kinda noisy.
Line 6 DL4 - Short loop times, very cool loop manipulation, 1/2 speed, high speed. One shot for playing with a beat. Mine has been good, but apparently very flimsy.
TC Electronic Ditto - Long loop times, single switch, really good for practicing not so much a performance looper.
I have my eyes set on the Boomerang mostly because the most fun I had looping was with the DL4, and the boomerang seems to be that and more, however I will never get rid of the Ditto. It's to useful and the footprint is perfect. These can get pretty cheap on the used market.