Dude, when I permanently switched from Mountain Dew to diet Dew my freshman year, I lost ten pounds. I never drink sugar soda anymore.
It all depends on how much you consume. A few a day is fine, really. The FDA is pretty anal about what they allow, so if asparamine really presented a danger, they'd probably make beverage companies use something else.
Something interesting to contemplate: Soda is carbonated. You're ingesting CO2. Your body breathes in O2 and exhales the waste byproduct, CO2. So there's probably something inherently not exactly healthy there, lol. Further, I've read quips here and there about the effect of carbonated beverages on accumulation of muggawugga in the lower intestines. Someone here probably knows what I'm talking about. So, as with anything, moderation is key. You'll be fine.
Are you on cycle?