When I first watched Season 5, I HATED Dawn. I thought she was so annoying, and like LMLTF said, I sometimes thought some of her tantrums are that of a 10 year old. However, the circumstances were definately different for her, especially when she found out she was the Key and all. She was a teenager and a little sister. I mean, how many little sisters aren't annoying?
And, she was what Season 5 was all about. She needed to be there. I agree with a previous poster; however much she annoyed me, I wouldn't have it any other way.
Anyway, once I rewatched the season, she grew on me, like other posters have said. Honestly, I don't think I'll ever particularly like her as a character, especially in that season, though I didn't mind her in Season 7. But, she's necessary, she provides a lot and is great for some of the other characters, developing them a lot. The most obvious example is Buffy,but there was a lot of impact on Spike and some of the other characters too.
And, again like others have mentioned, it's great for Buffy to have some real family after Joyce dies, and Dawn does a lot for the episode 'The Body'.