I don't think that the 80 mg of caffeine in red bull is an issue. A decent cup of coffee contains around 100 mg, but coffee is much more variable so I don't wanna get flamed. The biggest concern should be with the sugar content, but since you get the diet ones you should be fine.
The biggest issue I have with red bull is the price. They are so expensive and know they have such a huge part of the market it is sick. There are cheaper ones that are much more potent out there. I'm a huge fan of Rockstar because of the much higher caffeine content, 200 mg per serving and a can is 2 servings. That is around 2 bucks a can though; so I have recently been buying Monster which doesn't list the caffeine anywhere but you can get it for around 13 bucks or so a 12 pack at sams club. Red bull was like 30 bucks a 16 pack or some crap and is literally 1/4 the size of monster.
I have yet to see a study that says caffeine is bad. Most of the negative studies show that it raises your blood pressure, but that isn't horrible in moderation. Powerlifting has shown to increase blood pressure like 5x or something, enough to have some people bust capillaries in their eyes. Any workout increases blood pressure temporarily, high blood pressure is only bad if it is chronic (from filled artieries or obesity) since the heart never gets to rest and experience levels of low blood pressure.