My mom likes to tell stories about how, when she was pregnant, she would always put her food in the microwave, press the "start" button, and run away as fast as she could so as to not irradiate the fetus.
Of course microwaves do not actually irradiate anything (even my mom knew that, on a rational level), but a lot of people still consider them vaguely evil. At the very least they don't enjoy the best reputation for producing healthy or delicious food.
But they may be better for the environment, at least in certain circumstances. As an interesting Slate story found, using a microwave for cooking small portions is much more energy efficient than using an oven. Making a single baked potato in an oven took 9.5 times as much energy as it did in a microwave; making four portions of baked potato in a microwave took 2.5 times the energy. The efficiency ratios are skewed depending on whether you have a gas or electric oven -- gas ovens produce fewer carbon emissions per units of heat than electric ovens.
Good to know, though until they make microwaves that can brown the top of a half-eaten casserole, I'm going to have to stick to my oven for certain reheating jobs.
How often do you use your microwave versus your oven?
Source