I have used a mayawrap for two of my three children. In my top 5 list of things you must have with a baby, my mayawrap sling is number one.
My daughter was 11kg at 12 months and at no point did I find her too heavy for the sling. She was being carried in the sling until I was too pregnant with her brother to fit her leg over the bump, not that she was too heavy (I was about 8 months pregnant).
I have found the versatility of the Mayawrap just amazing from the different methods of carry, to the ability to strap your toddler to a restaurant chair should they not have a baby seat.
The reason I got a Mayawarp in the first place was I watched a lady pop her baby in a sling, latch the baby on to feed, pick up her two shopping bags and get off the bus. I thought I have got to have one of those.
My son is still using the wrap and I just love the fact that he can see everything I can see and we chat constantly when we're out. He is now 20 months and his language skills are really taking off. I haven't needed to put him in a push chair because he is too heavy and so we are able to maintain a wonderful connection when we go out.
I don't have a back that is particularly stronger than anyone else, in fact I have had to call an ambulance out before now because I put my back out and couldn't move-I've never had a problem with the Mayawrap sling.
Sling style carriers like the Mayawrap have been used for thousands of years. The Mayawrap is based on slings used in South America. Women from Africa to Indonesia have used sling style carriers made out of pieces of fabric for generations. Personally I feel that this makes this style of sling a tried and tested one. The purpose of the rings is just to make the sling more easily adjustable.
Finally I know there is some research out there, on the internet, that has suggested carriers in the style of the Baby Bjorn, with the small area of material supporting the entire weight of the baby, may cause spinal compression.
Just a few of my thoughts.