Heh, I've bought the frozen pre-packed ones before.. they are pretty shit.. but good to take for lunch to work and still healthy and they are still a potato! Just pre-baked and frozen.
I'm pretty good at making them myself though.. even better than my missus
Poke each one several times - 7 minutes in the microwave.
Then slap on baking tray and I use some low fat olive oil to spray onto them - stick it in a pre-heated oven (200 degrees) for about 50 minutes - 1hour.... and voila... perfectly cooked and crispy as fuck jackets!
I don't particularly like any kind of butter.. I don't eat it at all.. but margarine is just horrible harv.. makes me wince just thinking of it... you know, margarine is not even yellow.. its a sludgy grey when its first produced and they bleach and colour it to make it look like a natural butter colour.
Anyway, I'll usually just have low fat cheese on mine with beans.. but given the choice, chilli con carne would be the answer everytime.