I watched The Simpsons / Family Guy crossover episode yesterday. It was okay, but not nearly as clever and funny as any episode during the first six seasons of The Simpsons. It was a full throttle crossover with the entire Griffin family ending up in Springfield and will likely result in a ratings spike.
The key to a good crossover is subtlety, and The Simpsons / Family Guy crossover was as subtle as a sledge hammer. I far prefer how The Wire and Homicide: Life on the Street crossed over with an appearance by Richard Belzer's John Munch.
Here's 30 seconds of Detective John Munch on Homicide followed by 30 seconds of Detective John Munch on The Wire. I have no idea why this footage is so dark.
John Munch, by the way, has appeared on 10 different shows, which is surely a record.