I just did the 2 week induction phase a year or so ago and lost half a stone during just those 2 weeks. I'm thinking of doing it again just to give my diet a quick start, but wouldn't do it long term.
I did plan to do it longer, but we ate out a lot and had guests and it was just too difficult, so switched just to a general low fat diet afterwards and did take me another couple of months to lose another 1/4 stone.
I had a terrible headache for the first few days and felt permanently mildly nauseous for 2 days as well. Didn't help stop me eating though, it was a bit like mild morning sickness, the type of sickness where you know you need to eat to keep it at bay, but it puts you off a lot of foods and the last thing you really feel like eating are the Atkins meaty fatty foods!