My husband and I love the Beco Gemini. I had been researching carriers while pregnant with our DS and had settled on an Ergo until a dear friend talked about her Beco Gemini. Comparing the two carriers, I fell immediately in love with the Gemini.
It has a higher back and safety buckles to hold baby securely. The foldable headrest is great for growing babies and for FFO (front facing out) carry. Unfolded, just attach it to the D rings for head support for your squishy infants; fold it down for older children neck support or to let your little one look out and about. The adjustable base is great too! Fold it in for when your child is little and unfold it when they get a little bigger. There is no need for any inserts!
As for comfort, I hardly notice that I'm carrying an extra 14 lbs with my DS. The weight is distributed evenly from my shoulders to my hips. My husband also finds the Gemini very comfortable and loves the adjustability. It is so easy for one of us to take it off and hand it to the other. We almost fight to see who gets to carry our DS in the Gemini!
Above all, we love how ergonomic it is for us and for our DS. We feel very comfortable knowing that his growing body is being supported properly, while our own bodies can carry him without getting achy or tired.