Well I tend to take them apart and collect the magnetrons and flyback transformers... I must have 5 magnetrons and haven't used a single one - but I keep them because I never know when I'm going to want a multi-kilowatt microwave emitter - or maybe build a particle accelerator...
The flyback transformers are always useful 'cos who doesn't love them some Tesla coils?
WRT acutal cooking there are a couple of cakes/self-saucing puddings I use the microwave for as they are perfect for emergencies. I can produce a sensible chocolate pudding for 2 in about 10 minutes, but it doesn't match something proper made with a real oven. I may be a astrophysics student, but the pure efficiency of a microwave is not the answer when it comes to cooking - having my food heated my jiggling water molecules is always mushier than if it were cooked properly...