Just to add to PP, my experience was identical. Used the BB for my daughter (with back support) up until she was about 10 months old but by then it killed my back. Used it again with my son from newborn, he is/was a big baby and so by 6 months was really too heavy so bought the Ergo. It is a life saver in HK where it is often really difficult to use a stroller, especially when taking my daughter to pre school as it is steps all the way. I can carry my 10kg 10 month old son easily in the Ergo. It can get a bit hot (I have the original canvas one not the Sport) but doesn't seem to bother him that much - if it is OK for me to walk around in, it is OK for him to be carried in the Ergo!!
My husband doesn't like the fact that there is no facing forward option and so he mainly uses the Ergo as a back pack and finds it very comfortable - much more than the proper backpak carrier we had for our daughter when she got older as that was so heavy, even without her in it! My son sleeps happily in the Ergo too, in all of the positions and it is great for public transport.