If you're shopping around, hit thebabywearer.com. You can find a good used condition mei tai for $30-50. I wore dd in a Babyhawk at 9m at Disney in the Sept heat without any problems. My back never hurt the whole week we were there, and she spent most of her time worn over being in the stroller. It worked out so nicely to not have to have her in and out of the stroller all of the time.
I only had a baby bjorn with my first dd, and I don't think I used it much after she was a few months old. Just not comfy enough. And as pp have mentioned it's not very good for their spinal development either. I'm sure short periods of time are fine, but for longer periods, I wouldn't use it.