I was on Roaccutane for about a year. I did get very dry lips and slightly more dry skin in general, but other than that I didn't really suffer so I was lucky. At times it did make me feel really unwell, just in general. Keep moisturising and you should be fine.
They will keep doing blood tests to check your liver function throughout your treatment. If you are a girl, it's also compulsory that you take birth control for the course of your treatment - roaccutane and pregnancy do not mix. I had to sign a disclaimer before treatment that I would take birth control and that should I get pregnant I consent to an abortion.
It worked perfectly for me, got worse for a few months and then cleared up in no time at all. It really is good stuff!
There is also absolutely NO link between diet and acne. Personally, I believe my qualified dermatologist and the regulated, scientific, controlled dermatological research over "moonbase".