hgc is right, but it's way more true for some people than for others.
The starches/sugars in a plain baked potato are
very blood sugar reactive, truly. The
glycemic index of foods measures the blood sugar/insulin reaction to 50g of carbs from foods (more info
here). On a scale where the body's reaction to 50g of pure glucose is 100, enough plain baked potato to provide 50g of carbs scores
85 or so (they vary). Table sugar is 59 and potato chips are 51.
The
glycemic load, which may be a better measure (it takes fiber and serving size more into account), is 26 where glucose is 50. On that scale, potato chips are 30. I'm sure various potato types will score differently.
A medium baked potato with skin offers 31 grams of carbohydrate, 3 grams of which are fiber, according to Fitday.com. So the amount of baked potato used to score the GI test would be a large one. Baked potato does have some good vitamins, like C and B-6.
For people with risk factors for diabetes, which is really a lot of people and includes me, high GI foods like that are a very poor choice. We want to keep our blood sugar stable throughout the day; high GI foods make our blood sugar soar up and then crash down. A half of a baked potato (smaller load of carbs) alongside some fat and protein sources (to help slow down absorption) is more manageable, but it's not something I would eat every day (nor are fries or potato chips).
But some people just don't have a problem with their blood sugar spiking and crashing like that. It's not as extreme in their case, or it doesn't bother them. Still, lower-GI carbs are considered better for your health overall, due to the long chain of reactions the body has to high-carb foods (triglycerides going up, etc.).