I've used my Moby from day one, and I still use it with my 15 month old (25lbs, about 3ft tall). A friend has a 3 year old she wears with it (he's almost 40lbs...skinny lil guy). Definitely worth it!
I've carried two babies (mine and a friend's) at once, nursed while hiking wearing it, and it folds up easily into my purse so I can take it everywhere. DD hated being swaddled as an infant, and she still liked the Moby. Now when we're out and she gets tired or if she's sick or having really bad teething pain she wants to be carried in it. A different friend has a homemade Moby, works 50+ hours a week, and still gets lots of use out of it. The wrap is the only way her husband can get the baby to sleep when Mom isn't home. No adjusting needed, it fits both of them.
I also read that Moby has a smaller, ergo-style carrier called Moby GO...I tried it in a local natural parenting store and I'll definitely buy one if/when we have another so I'll have two on hand.
(I don't retail the brand, promise..I'm just sold on it!)