In reply to ollieollie:
We have two, and both are good for different reasons. We have a Vaude Jolly Comfort 2, which is a 'proper' carrier for babies over 6 months. It's comfortable for both me and the child, and has a very handy waterproof rain canopy, which is good in the wind too. There's a decent luggage space to carry all you need for a day out too.
The other one we have is a Littlelife Ultralight convertible S2. This gets far more use than the other one, both on days out about town and in the hills. The advantage of this one is that it zips up and looks like a normal biggish rucksack, then you unzip it when you want to put the child in. We've found this much more useful when the child starts to walk a bit, as it is more compact to carry about. The disadvantage is that it doesn't protect the child as much in poor conditions.
One tip I would give, is to try them out in the shop with your child. I found that some felt really uncomfortable to carry, and others didn't adjust small enough to secure our 6 month old (she was very small for her age, to be fair).
Post edited at 16:44