First of all, you may or may not have a legitimate complaint (I wouln't know since I've never heard of this restaurant before), but this letter is poorly written and it makes you seem less credible. For example, use exclamation point sparingly, and NEVER use multiple exclamation point, or question marks and exclamation points together. Also, the YUCK! YUCK! YUCK! is not necessary and unprofessional. Simply saying that you are dissatisfied or that you dislike the sodas should suffice. Also, 2000% is way too much hyperbole for a credible letter. I'm not sure why you would compare them to Taco Bell, anyway. Are the two establishments similar?

Also, the comments you make about the quality, service, and food are complimentary, which I'm sure they'll appreciate. But it seems to me that if you're otherwise pleased with the restaurant, the paying a bit more would be worth it.

I suggest rewriting the letter when you are less angry, and be sure to explain your complaint in a professional, less hostile, more coherent manner.