Macau was the first (and last) Portuguese colony in Asia up until 1999, when it was formally handed over to China. As a Special Administrative Region (SAR) of China, Macau is pretty autonomous under the "one party, two systems" principle. Hong Kong is also governed under this principle; these two SARs have capitalist economies in an otherwise socialist country. Because of Macau's special governing system, it's the only place in China where gambling is allowed.
There's a lot more to see in Macau besides the casinos, though. The Portuguese were in Macau for four and a half centuries before China took over and they definitely left a mark. The old buildings, churches, and narrow cobbled streets have an old world charm that makes you forget you're smack in the middle of Asia.
Often, the old is situated right next to something new, like an enormous flat-screen tv mounted on a building across from the Ruins of the Church of St. Paul, or a beautiful little cobble-stoned alleyway with a McDonald's in it. It makes for an interesting mix of things to see and do.
Many street and building signs are in both Portuguese and Chinese; I could read the ones in Portuguese. We had lunch at a Macanese restaurant where I eavesdropped on conversations in Portuguese, too.
Even if you go just to walk around and have lunch, a day trip to Macau is worth doing. You don't need a visa and HK dollars are accepted everywhere (the exchange rate with the Macau pataca is about 1:1). Here are a few more photos of our trip to Macau.