I'm 5'4" and have been riding a Giant XTC 29er all season and I LOVE IT! I kept my 26-inch FS bike, thinking I would want to suspension sometimes, but I rarely ride it because my HT 29er just climbs so much better. (The ride is a bit rougher sometimes, but the speed makes up for it.)
I feel like I'm right on the border of being too small for my particular bike, because the handlebars are still a tad high even with a flat bar and the stem turned upside down. However, I have short legs for my height, so it might fit a shorter but longer-legged girl as well as it fits me. The standover height may be a little high for shorter girls, but how much time do you really spend straddling your top tube? I find that geometry is quite comfortable once I'm in the saddle.