I've had two. They make the poop factories that we call freshwater plecos look like amateurs. Then again, given how much algae they eat it has to go somewhere. The first decided to clean the algae off of my MP60 one day a month into its tenure and never left. It didn't spaghetti like a nem, but it didn't survive either. I didn't see a huge dent in my algae, but at that time I was pulling a softball of compacted hair algae (yes, really) out of the tank every couple days. I was also nitrate dosing, but more importantly I think I was leaching a ton of phosphate out of my rocks and the nitrate dosing was keeping the algae from starving the coral while letting the phosphate slowly lower.
The second I think I had in my sump. It lasted a month or two and died of unknown causes. Whatever the cause, a shortage of hair algae was not at fault.
I knew another guy who had one in a nearly pristine 75 for years. No idea what it ate other than the nori he put in, but it kept the tank clean apparently.
I've heard many tales of how they can poison tanks, but I've never seen it happen despite knowing a few people (myself included) who have had them croak in tanks, so I wonder if that's a real risk or wives tale at this point.
Based on how easily my two died (while everything else in the tank thrived), I'd be a bit nervous about having a formal loaner.