{I understand her purposes for being introduced to the show, but I think that Joss may have made it a little complicated for my tiny little brain. Although, maybe he needed to make it complicated for it to make sense...hmmm}

It was brilliant! I absolutely loved her introduction in Season 5. You know she doesn't belong but everyone's acting like it's completely normal. It was such a unique way to introduce the story arc for that season. I didn't think it was that complicated. I mean of course you don't know who the hell she is right away but that's the whole idea. And you know it's going to be revealed eventually...
Plus, I really love what Dawn brought to the show. She gave Buffy a kind of maturity that I'm not sure she would have achieved otherwise. I honestly can't imagine Season 5, 6 and 7 without her. She really became essential IMO.

I found Dawn very annoying but I found the storyline intereseting, Glory was my favorite villian

I have always felt it was a lame gimmick and Michelle pulled the fat from the fire. With a lesser actor it would have been the usual disaster when you add a kid to an ageing series.

She's pretty annoying but I just love the final episodes of season 5 and she's like indispensable for that part.. Anyway I think she is just terrible at the beginning of season 5, but she got better and better (even if I still don't like her that much)

She's so annoying. Always complaining about everything.

It showed us a different role of Buffy, what she could be as a sister and a surrogate-mother later but Dawn was truly annoying.

she's not my fave but I can't tell I hate her. At the beginning, yes she's very much annoying person, but since her relationship with Buffy gets better and she finds out she's the key I like her better. Still I dont think it was a good move to bring her on the show as Buffy's sister...

I like the idea of a mystery sister from nowhere, but I find Dawn's character a bit mediocre. Maybe I'd like her more if her Key-ness remained relevant after season 5, or if she had any other stories after that season (not counting kleptomania or a brief assumption that she was a Potential).

Confusion was part of the whole idea. You were meant to think "where has she come from?" and possibly even remember that it'd been mentioned previously that Buffy was (originally) an only child.