Ask HN: How do you feel about fixed navbars?

Lately I've noticed this big trend of sites having fixed navbars when you scroll down, and then an even bigger fixed nav appear when you're down the page and scroll up a little (like wired) What are your thoughts on this? I personally hate it and even find myself deleting the nav element from the source. It just seems like a 'cool' design feature that is not practical. Taking up ~10% of my screen is much more of a hassle than me scrolling to the top if I want to navigate the site. Do you think it's practical? Useful? Or a waste of space