My DS (8 months and 23+ pounds) and I love our Ergo! I vacuum my entire house with him in it! I prefer the back carry because I find it more comfortable and I can get things done around the house (like making dinner, folding laundry, etc.). He's very comfortable napping in both the back carry and front carry position. Just a couple of weeks ago, I wore him for about 3 hours (he was asleep for about 2 hours) while at my DD's preschool picnic.
I would stay away from the Bjorn (even though I loved the front carry option with DD) because it is not very comfortable (in comparison to the Ergo) and the weight limit is 25 pounds. The Ergo goes up to 40 pounds. Others have mentioned the Beco and Babyhawk which are similar, but IMHO are much more fashionable. They were beautiful, but I preferred the simplicity of the Ergo myself.