Is club soda bad for me?

Not at all: Club soda is generally only carbonated water and is no worse than water, aside from the gassy side effect. There are no health ramifications and it is good for you.

Too much of anything: Excessive intake of anything can be a problem, but this is reasonably safe. There may be a bit of saltiness and if you find the gassy properties uncomfortable you'd not want to drink it anyway. Huge amounts might raise your acidity thereby reducing your calcium absorption (strong bones) and tooth enamel may suffer some slow damage. So just be moderate - always a good idea anyway. Best!