I have one and love it! Joshua just turned 1 and is about 20 lbs. and I can carry him comfortably. I can also carry Pacey in it at 27 lbs., but not for as long as Joshua.
I have a bit of discomfort in my shoulder/neck area where the strap goes, but not enough to REALLY bother me. It's a million times better than a front carrier...that's for sure! It doesn't really leave both hands free (I think it advertises that it does) because the baby is in the way (you can see it in the picture), but it doesn't break your arm like carrying baby does, lol. The straps you tighten don't really have anywhere to go, but you can just tuck them under, no big deal.
I think it's extremely easy to put on, get baby in, and get it tightened. Once you do it a few times, it's nothing! When we go to the store, I just buckle it around my waist, then get Joshua out of his carseat and sit him on my hip (just like I would if I were carrying him) on top of the carrier, pull the carrier up and over my head, and tighten it. Believe me, it sounds more complicated than it is...it takes me 30 seconds! Plus the instructions are right there on the carrier if you need them. Oh and it's just as easy to get baby out of and take off.