I'll pipe in as well. After doing my research, I found the Ketogenic Diet and it was fantastic. I lost 100lbs in a year (from 299 -> 199)
I'd highly recommend reading 'The Ketogenic Diet' and 'The Rapid Fat Loss Handbook' both by Lyle MacDonald, particularly if you have a scientific interest in it, as well as a nutritional one.
In response to your question, I did not feel like I lost any mental faculties during it. The first week was fairly difficult, hungry and all, but after that, it's pretty easy. Best of luck!