I don't have a microwave--we've had an Advantium since they were introduced. I use it many, many times a day. I use the 'straight' microwave feature for: melting butter, softening butter or cream cheese to spreading consistency, cooking vegetables, defrosting, reheating leftovers, cooking bacon, melting chocolate, making white sauce, etc etc etc.
I use the speed cook feature for: baking cookies, potatoes (it makes REAL baked potatoes in 8 minutes--not steamed/nuked ones), pies, casseroles (done in 12 minutes with a crisp brown crust), reheating pizza or leftover sandwiches, muffins, cupcakes, cakes, cheesecakes, other desserts (like rice/bread pudding, etc) and pretty much anything else that can be baked, broiled or heated in a regular oven..
The two things I WILL NOT do in a microwave or Advantium are: popcorn--which is very bad for your oven and voids the warrantee (yes, I know they put a popcorn setting on many microwave ovens, but it's still one of the worst foods you can cook in it--eventually ruins the 'works') and heating liquids like water for tea--very, very, very dangerous. It's been proven that on occasion, the molecules of water heated in the microwave will become so excited that the water will 'explode' at you when the oven is opened, causing serious burns. Doesn't happen every time, but it's a serious enough danger that one really shouldn't use the microwave for boiling water, heating tea, etc.