I have the ergo sport and have used it from birth & love it! I use it everywhere I go, prefer it to the pram.
It's ergonomically designed to support bubs body (they're in a sitting position rather than hanging which is what babies do in most other carriers).
It comes in 3 diff styles we got the sport because it's lighter and dh found it to be the most comfortable of the 3.
The weight is carried by your hips not your back so you can wear it for ling periods without a prob.
Love it