OMG! I didn't even know diet cherry coke existed! You lucky Americans!
I tried giving up all sorts of cola at new year, and haven't touched one yet! I used to drink a lot because it was something sweet with no calories, but after a while I realised that they all taste pretty bad and are full of chemicals!
As mauvaisroux says, they are full of caffine, and I heard that the acid in cola can hinder the absorption of calcium in the gut, leading to osteoperosis. So don't drink with milk!
I think if it's helping you with the weight loss then cool, but I wouldn't drink more than a coupla glasses a day.