I use Flat. In fact I had to change it back to Flat the other day because it reverted Threaded.

I visit the site roughly once a week, and then make an effort to become current. This has the benefit that clicking on a thread title takes me to the first unread message in the thread, and it's easy to see all the new stuff. Since the upgrade a few months ago (and getting my own broadband modem replaced) bandwidth hasn't been a problem for me.

Generally the threading structure isn't important. If it matters, I can generally figure it out from the author names, if necessary clicking on them. (The "re: author" part of the post title links to the post being replied to. I'm guessing not everyone realises that.)