My cat Angel was diagnosed with diabetes about a month ago. We took her to the vet because she was urinating heavily, drinking an amazing amount of water, had low energy and interest, and was almost frantic about food. For an 8-year-old overweight cat (she's an ex-stray with food issues and not a naturally high-energy cat) that was rather obvious, but blood tests confirmed it. We caught it early, started treating it early: changed her diet and started insulin injections which she's tolerating extremely well.
She surprised us with at least ten minutes of (for her quite vigorous) playing this morning, minutes after having a blood sugar level test taken - you can still see the shininess on her right ear from first vaseline to help get the drop of blood necessary, then polysporin on the tiny puncture afterwards. She doesn't care for the process, but obviously doesn't find it too traumatic, since she stayed on the bed and then started playing!