I used one for 18months - til dd was 30lbs and i got a back carrier instead. I have a bad back that flares up occasionally. the first few times I used it, i was sore, but it was because I was overcompensating, and still sticking my hip out.
I saw one on the net that had storage on the inside of the seat, which the hippychick seat dosent - will have a wee check for you.
I certainly found mine helpful but only used it for a few months as I got pregnant again and DS1 had to learn to walk everywhere. I used it for DS2 as well but then again only for a short while. If you can try one out would really reccommend it - perhaps borrow one or go to a shop and have a go (John lewis - where we got ours from).
Yes it helped with my back pain. It sort of gave support and it reminded me to do everything carefully with stomach muscles sucked in etc! I didn't use it that much more for going up and downs stairs and at a singing/music thing doing the grand old duke of york! My son was a real heavyweight so I didn't use it for day out etc.
Yes, it really does spread their weight around rather than concentrating it on your hip/lower back. I put off buying one as i wasn't sure it would fit around my size 20 middle, but it did, with room to spare too. highly recommended.