I personally enjoy reading most people's sigs. I also use them to form an initial impression of someone, although my opinion will change based on their posts. But I don't think about it much, I just scroll past the ones I don't like.
The ones that I dislike are those that distract from the conversation - if they abuse color, are
Extra Large, or have pictures or borders around text (I really appreciate sblocks though

). For those of you that have sigs like that, before you get defensive, remember it distracts from the points
you are trying to make, too.
If it's a quote or reference I don't get, I don't pay much attention.
Mine I chose back just before Dave Arneson died; I'd been searching for a Gygax quote to put in my sig but I couldn't find a good one that I hadn't seen in someone else's sig.
So I figured, what the heck, an Arneson quote is good too. And it's nice to know that new DM/GMs have been thinking that since the beginning - and being proved wrong (in a good way) by uncooperative players LOL.
I think it is important if you're going to put a quote in your sig that you provide a little context if it's not apparent and needed in order to understand the context. Like the broccoli quote - I remember the ads, now that someone's said something, but not just from the quote by itself.