I've heard they aren't held to the same quality assurance testing of official Lego sets so they can be either difficult to assemble, flimsy when built or otherwise not quite what you'd expect from "real" Lego, but they are some of the most amazing sets I've seen. Really cool!
Obviously I've mentioned collecting Lego plane sets before, so this is the Bricklink set I really want:

Luckily there's nothing unique or rare in it so I can piece it out for about half the price, haven't started collecting the pieces yet but I think I will soon.
Edit: excited for this one!

Feels too good to be true. Been looking for this one for a while but didn't want to pay £90 for a BNISB one, it's kind of Model Team-y but a 3-in-1 and the other builds look great too. Here's hoping it is actually complete, in good condition and that it arrives, I assume the seller wanted a lot more but it only had three watchers and one bidder.
Ediiit: I legitimately struck gold, I bought a 31096 Twin Rotor Helicopter for under £10 and not only is it genuinely complete and in very good condition as advertised, the manuals are immaculate too. A really, really good find and I'm glad I didn't listen to the voice in my head saying "stop buying sets you already have" - it's still 600+ pieces for under £10! I was going to let my son have this now to go with his similarly huge Jungle Explorer helicopter but the condition is too good, I can't do it.