The Snuggie is basically intended to provide the warmth of a blanket without restricting your ability to perform activities that come with normal blanket usage.
It's basically a robe you wear backwards. Unlike a robe, however, there is nothing to tie around your waist to keep it on. The solution is that the top of the Snuggie is large and rectangular. This way, it basically hangs over your shoulders and the weight and friction of the fabric on your shoulders and back keep the Snuggie from sliding off.
So, since the Snuggie is a hybrid of the blanket and the robe, how does it compare to its predecessors?
The Snuggie works best when reclining, as it does indeed offer warmth and mobility. The fleece material, along with the bunched up fabric under the neck, give it the cozy feeling that a robe can't offer. When walking around, however, the Snuggie provides no warmth for the back, and may fall off. As a result, the Snuggie isn't always the best choice, but when lounging on the couch watching TV, the Snuggie performs remarkably well.
