I drink diet soda every day. I have read/heard from multiple surgeons that the idea of carbonation stretching the pouch is only a myth. I find it much easier to take my vitamins with carbonation, and my common sense says it's good to have the additional acid of the soda with my pills since I have less stomach acid now due to the smaller stomach (but again, that's my common sense talking, not an actual doctor).
I just can't drink much soda at a time. I take vitamins 4 times a day, and it will take me about 2 days to finish a single can of soda.
Of course, I would recommend not drinking soda if it causes pain or discomfort, but other than that, I don't think it's really so evil (unless you're drinking regular instead of diet, or if you're drinking so much soda that you're not getting in some healthier beverages). I choose diet sodas with no caffeine and no phosphoric acid *that is the acid that is believed to leech calcium from your body, and it is found in cola-type sodas), so I usually have diet ginger ale or diet orange soda.
Of course, I also drink water (preferably SmartWater for the electrolytes since I have been dehydrated in the past and wish not to repeat it), and I like the new flavors of sf drink mixes lik cranberry lemonade, white cran peach, and cran apple.
Highest known weight: 324
Weight on morning of DS surgery: 308.5
Got down to about 185 before pregnancy;
Benjamin David born March 24, 2012, 7 pounds 11 ounces