I find them really hard to follow. Threaded commenting isn't so fun when you can't collapse a single comment's thread. Instead, if I want to know who is being replied to, I need to place my finger on the screen and scroll up, seeing on who's comment it lands. That tells me which comment the current one is replying to.

4chan (well, imagboards in general) have a nice system where you can reply to multiple people at once by referring to the post id. Top-level comments are usually represented by referring to no comment, or by referring to the OP's post id. I find this much, much easier to follow.