I had a Baby Bjorn for my DD and encountered the same problems as some of the posters in this thread. My DD did not like it when she was very small and by the time she was 15 lbs. it hurt my back.
I really wish I had bought a sling back then, especially since my daughter had colic. I did not do any research on carriers or slings a couple of years ago with my first pregnancy. I recently sold my Baby Bjorn on ebay and purchased a Zolo sling for baby #2 who is due any day now. After reading the message boards here for a couple of weeks I decided I wanted another sling as a back up and just purchased the Kangaroo Korner Adjustable Fleece Pouch. I would definitely recommend that you check out www.kangarookorner.com because they rate many slings and pouches there and tell you all of the advantages and disadvantages of them. There are so many different carriers/slings out there so you should definitely read the archives here and check out the ones that best suit your needs.