Getting 150 calories from a Twinkie certainly is less beneficial than 150 calories from oatmeal, for the exact reasons you describe, but they both give your body 150 calories to use (or store...).
Hacker's Diet is the best way to lose weight IMHO. It explains the basics (consumer less calories than you burn), and offers some good strategies for eating and exercise and geeky tools (inlcuding a web-based tracker) to aid in your descent into fitness. I lost close to 30 lbs on the "diet" and while it wasn't painless, it was pretty straightforward. I did gain a good amount back 2 years later when I quit smoking, however.
That aside, I do want to correct something -- most clerks (at least the good ones) actually memorize nearly all the fruits and all their code numbers. Check it next time you buy produce, they almost never look at the code before keying it in. Even with apples. It's actually pretty impressive how much cashiers at grocery stores have to memorize (they get paid better than you'd expect, too.)
Pascal is not a high-level language. -- Steven Feiner