We bought a Baby Bjorn and absolutely loved it for the first few months. After 5 months though we had to graduate to something more ergonomically sound. We used it initially when our daughter was only 3 weeks old and she loved it- we used it for walking all over town and hiking all over the downs and she was so sung and happy. I have to say baby wearing is a fantastic way to transport baby! It totally beats pushing a buggy on a busy Saturday morning trying to negotiate through the crowds!

When my daughter reached about 15 pounds it started to become difficult to carry her in the Bjorn. There is a crossbar on the backstraps that are a little fiddly- if you don't position them exactly where they need to be the carrier will really pinch your shoulders and it's not comfortable at all. I also noticed that the heavier she became the more often the fiddly crossbar would actually move while we were out and it's near to impossible to adjust it on your own unless you take the baby out which is a big hassle really.

The baby is also "suspended" basically by their crotch which I can't imagine would be super comfortable.
The pros are that the baby can be facing in towards you or outwards to see the world (after they can hold their head up on their own). My daughter really liked this aspect as she became more aware of her surroundings.