there are many reasons why the BB is inferior to many other baby carriers on the market.
babies are born with only one curve in their spine (as opposed to the adult 4). the baby bjorn is a poorly designed carrier that allows all of babies weight to hang from his/her pelvis which puts undue stress and pressure on the pelvis and spine. better carriers are mei teis, slings, wraps, and other soft structured carriers (SSCs) like the beco and ergo. these are much more comfortable for the wearer too.
as you baby gets bigger, back wearing will be better for YOUR spine, and you can't do that with a bb. check out babywearer.com for everything you ever wanted to know about carriers, reviews, forums, how tos, for sale/trade, etc. there are so many beautiful, and far more comfortable carriers on the market that give you way more for your money's worth.