I love the RC diet principles: so much easier than the other 'big guns' of WW and SW (only in my opinion - the other diets themselves are sound: I just found them fiddly and annoying).
Diva's right about the RC 5% rule. Foods that contain less than 5g of fat per 100g have the thumbs up. It's easy to check this on the nutritional labels shown on all packaged foods these days.
In addition, there is a limit on starchy carbs so you have a calorie guide and low GI foods are encouraged.
I lost 6st on RC a few years back: it's a great diet and the exercise class is fab. Unfortunately, as I had 12st to lose, I lost the plot and got disheartened at the slow (but steady) pace. Like a twit I gave up and regained 4st over the following 2 years. (Cue Cambridge!
I would definitely go back to RC after I reach goal but unforunately, the woman who used to own the franchise in my town has given up and so the nearest class to me involves a trip across the harbour or a long drive round!
Anyway - I'd say go for it if you're thinking of giving RC a go.