We have a backpack. It's good for little ones too big for a Snugli type thing.
I wouldn't recommend it for the parks though. Like mentioned in the previous post, it's kind of a pain to keep taking baby out and putting him back in.
It's much better for something like the zoo, where you want your little one to be able to see stuff, but don't need to get him out repeatedly.
We're just going to use a stroller.
Plus, you will get VERY hot and 25-30 lbs on your back for the day takes a toll.