We've got a Macpack Vamoose. We've used it from when DD was 6 months old. I now find her getting a little heavy for long walks, but DH absolutely loves it and carries her in it for hours on end.
I'm a big fan of slings, moby etc, but I found the backpack very good because it has got a proper harnas which adds to carrying comfort. It is also very handy for when we go to the library or shop because you can put quite a lot in it.
DD loves it and often drags it out of her room when she wants to go to the beach

.
It was very expensive (I think it was trhe most expensive item we ever bought for her

), but it's been well worth it, as we live by the beach and used it so much in the past year. It was also invaluable when we were on holiday.
Oh, and of course it's great that DH can carry her (instead of me). (My DH doesn't like using a sling, but he's a keen tramper and loves his backpacks).
Btw, you'll probably be able to get a secondhand one on TM.