Everlong- I think it's because she likes it as well as it being uncommon, clearly the OP doesn't think it sounds dated and boring.
At the end of the day I think picking a name is a VERY important part of being a parent. But you have to go with what you like not what other people like- you're the one who has to say name for the rest of your life! There's no guarantee that the child will like the name, whatever you pick, and there's no guarantee that if you pick a popular name it won't suddenly go out of fashion/ there will happen to be barely any in your area, or that if you pick an uncommon name that it won't suddenly become really popular (My mum hadn't heard a Jacob for years until my brother was about 2 and suddenly they all had his name :P).
So just go with what you like providing it suits your family eg doesn't sound silly with the surname, your sister doesn't have a child with a similar name etc
However, the general consensus is that Joanne is a boring, old fashioned name, so you have to think about whether you really want other people to think that when you tell them what you called your child and if you think your child is likely to wish you hadn't called them that and want a different name. You also need to think about potential nick names. If she's called Joanne but you always call her something she likes, for example Josy or Jo or Ann or Annie then it won't matter too much.
SO yeh it's your choice but while you shouldn't feel pressured to pick another name if you like it, you should bare in mind peoples reasons for not liking it :)