I'm kind of mixed on it, to be sure.
First of all, they can be noisy if you don't get the gain right. But you can usually (I won't say always -- usually IME) dial that out. It is fussy, that way, though. When people say 'Rangs are noisy, I suspect it's this issue, and they didn't take the time to work through it. There's a pot and a switch that control gain levels.
As far as sound quality goes, they aren't pristine like Boss can be. For that matter, the Echoplex Digital Pro isn't that great either, nor is the (original) Jamman. None of these devices are really studio tools (you have Live for that). On a stage, through your amp, I have never had any issues with sound quality. The Repeater (if it is ever released) will have modern specs.
The core issue, I think, for most loopers is "can you interactively control feedback of the loop?" Echoplex = yes. Lexicon Jamman = sort of but I don't remember how it works. Repeater = yes. Boomerang = yes, in a sense. DD-20 = sort of, as I understand it. All the others = no. (The TC 2290, the Eventide products, and, oh dear lord, Kyma are outside of my experience and cash flow.)
I have the Echoplex now, along with a TC D2 to do the ambient washes (which it does quite well). The problem with the Echoplex is there's too much, and you have to deal with its very fussy MIDI implementation or poke a million buttons on the front panel to get at it. (I hate MIDI.) If the Boomerang people had made the little roller thingy feedback instead of volume the whole debate would be closed in my opinion. It's got the very best interface of all of them, no bending over to twiddle knobs, easy shifting between its two loops, a nice "long delay" mode, the best reverse mode. I suspect I'm probably going to go back to it, although the Repeater (if it ever ships) is going to kill it in every category. The EDP will still be better than anything for mangling loops if you like that glitch-core stuff, but it's showing its age for sure. (It was designed, what, 30 years ago and hasn't really been significantly updated since.)
Where's Steve Lawson? He knows more than I do.