We bought this carrier when my LO was 5 months old and it has been our favourite since then. DH and I use it more than any other carrier.
It is easy to put on and the two-handed safety clips are simple to use.
We haven't tried it with a baby younger than 5 months but, with the adjustable base, I think it would work well for a smaller baby. It is still going strong 13 months later and I can comfortably carry our 17 month old.
I have tried a back carry with this carrier but I find the back support a bit short to securely hold my LO who does tend to fling herself around a bit. But this is the only con for me.