Yuck!
Sorry- I just hate the taste, and hate the smell even more- someone opens a can on public transport and the whole carrage is filled with redbull sickly chemical stench!
I'd say they can't be that good for you, a shot or two of caffiene isn't bad but to have them every day may give you a caffiene dependancy which could be at best hard to shake and at worst hamper your training by undermining you, making you believe you cannot get by without it and making you feel you need to rest alot to catch up on what you did do when you took the drink.
I also think they must have tonnes of sweeteners in- they taste overly sweet and this opens up a whole can of worms about sweeteners and their cancerous effects, dependancy, screwing up the bodies hunger signals...the list is endless!
I'd say rough rule of thumb is little and not very often, you should be fine. Just please, if you live in London, do NOT drink this stuff on the tube/bus/train- it makes my life a misery! (It does actually make me feel sick/nautious)