In reply to Carolyn:
Fair point, I still use the Ergo occasionally when just taking the dog round the block and our girl is 19 months. In general though we stopped using soft carriers as both of us (and other family/friends) found that past about 12-14 months structured carriers were more comfortable if any real distance was involved but that is very much an individual thing. Also I've found that when she is struggling/kicking/dancing it has much less effect on me if she's in the Macpac rather than the Ergo!
Another factor that pushed us was the lack of good weather proofing for soft slings. We considered just putting the wee one in her rainsuit and letting the sling get wet but it just didn't seem sensible. While she was young we could both just about get her in the sling inside a jacket while it was raining but that doesn't work when she's on the back. The Macpac that we went for has full covers.
Luggage space is also a big plus when we're off for a day walk without the wife, I only tried one real walk with the Ergo on the front and a bag on my back, felt like a year out backpacker and only last about 3 miles!