I've been using one of these for six months now, since my second child was born. My baby loves it and falls asleep in it while we are out and about. It is really comfortable, far more so than the baby bjorn I used for a while with my first child. You don't have to worry about complicated straps or ties, just throw it over your head, insert the baby and you are off.
It helped us to get over the period where the first child is still using a buggy and you also need to transport the baby, so you start to wonder whether you need a buggy that takes two. Those buggies are expensive and unwieldy, so I am happy I discovered this sling instead! It is easy to carry a baby in this while pushing a small one-child buggy until the elder child is old enough to walk pretty much everywhere.
It is absolutely fantastic in public transport and anywhere else that involves stairs. I felt much safer and it was certainly easier than dragging a buggy up and down stairs.
You should be aware that it is important to get the right size sling when using this. I had to go down a size about six weeks after starting to use one as the original one I had became too big - this sling helps you lose the baby weight!
£55 is pretty expensive though, I've seen it elsewhere in several places at around £40, and I was fortunate enough to get the first one I bought for £20 in a 50% off offer at Boots.