I'm the younger sister here (my brother is 4 years older than me), and I can go either way with Dawn. I wasn't that fond of her in season 5 (oh my GOD with the whining and screaming and tantrums), but on the other hand in Real Me when she's making her diary entries, I totally related to them, cuz I'm sure I wrote my little diaries and journals that way too when I was 14. It was kind of cute. I also related to her liking Xander and Spike because they were older, and cool, and therefore hot. I liked her a bit more in season 6, but not by a lot. I think I related most to her grief for Buffy (the scene where she goes to lay down with the BuffyBot is TEARJERKING, every time).
Season 7 however, I loved her. She had matured, she was witty, she'd fallen into the role of Watcher Jr. beautifully. I think the only time I really wasn't fond of her was in Him when she tried to kill herself (I know, I know, SPELL with the crazy-making, but still, it was sort of typical of younger Dawn), and in Potential when she thought she was going to be a Slayer and she goes and hides in her room and then climbs out the window. That ENTIRE scene reminded me of Season 5 when she does the same thing and ends up at the hospital. It completely irritated me. LATER in the episode, I had no problem with her, but that one scene just didn't sit right with me. I suppose that was a lot of information to take in at once, and naturally, that would be fairly freaksome, but it seemed like she was being the overdramatic teenager again.