I'd love to be able to switch back and forth between genders, though I wouldn't want to live my whole life as a girl. I like my man-bits, dammit! Seriously though, sometimes I think girls have it so much easier than us guys (a misconception, I know, so don't call me out on that). For example, dating. If a girl is interested in a guy she can give out subtle hints that suggest to the guy she may be interested. It's harder for a guy to be able to do that, though, and as a guy I know I've missed several big opportunities in the past (more than I've taken, at any rate) when a girl has hinted she's interested in me and I've not realised until later. It's little things like that, and trying to understand what girls are truly like and how they think, that would make me love being a girl, even just for short periods of time.
To be honest, I actually hope reincarnation or Spiritualism and whatnot are real, because if so then I'd love to be a girl in my next life...