Fairly low compared with cheddar. But even better if you buy the low fat varieties. I like Sainsbury's Be Good Greek Styke cheese which is 182 kcal per 100g and 11g fat per 100g.
(Cheddar is about 400kcal and 35g fat per 100g by comparison)
I wouldn't but low fat anything. Those products are usually filled with more salt,sugar, and artificial nasties...and as such are totally counter-productive. Eat real food.
//Wash it down with Diet Coke - that nullifies all the calories.// Hopkirk- that sounds a pretty disgusting combination! You should accompany feta cheese with a fairly robust, no-nonsense red wine, I would suggest a Chianti or a Rioja.