I work at home too, and it has its pluses and minuses. On the plus, gas as mentioned and on the minus, the social side. Being able to set your own schedule can be a double edged sword. While the flexibility is great, I find myself putting in far more hours a day than I would have if I was working a plain old 9-5 job. It's just too easy not to step away from it, especially if I'm building websites and get engrossed in it and forget all about dinner.
Finding balance can be a real challenge but then I don't miss the office politics, that's for sure!