J.,
I have an Ergo Baby Carrier and totally love it. My son weighs approx. 21 lbs, and he still fits comfortably in both the front and back position. Since he is one years old, I generally stick to the back position, unless I'm attempting to nurse him while doing something such as prepping dinner.
I am pretty much a baby gear junkie. I admit that I own 4 different carriers. I also own a hand-me-down Bjorn, a Moby Wrap, and a Maya Wrap. Since my son is a lot bigger now, the Ergo is really the only one I still use. I have literally done tons of house chores with my son on my back. I also like that it is easy for my husband to use. He actually carried ds on his back last nght, while we took a little evening stroll. As I've mentioned, the Ergo truly reigns supreme.
What I like most is that the Ergo allows DS to pretty much "mold" his body to mine. He seems to be very comfortable, and is always super peaceful while in the Ergo. I also like the fact that once you adjust the straps and waist band, you really don't feel any pressure or heaviness.
The only thing I don't like, is that the Ergo looks kind of "Granola Crunchy". I've seen better colors than the camel color that I bought, but that's really my only complaint.
As far as the weight limit. I've literally seen a 7 year old or so ride in an Ergo. The kid was playing in the park, his mom called him over, he jumped up on her back, and she strapped him in. I personally don't plan to use my Ergo quite that long, but the woman was walking fine, and the kid seemed really happy. On the other hand, maybe I will.
I really think you'll love it. Try checking Craiglist to, just in case. Best of luck to you and your new crawler.