Okay, so Placencia sounds like a good place to hang out and dive. Sounds like some good places to eat too. So, how's the night life when compared to AC?
I haven't been to AC, but I'm going to go out on a limb here and say it probably has more night life than does Placencia. AC's bigger, meaning more people, more bars and more "going on." Placencia has only a couple of nightspots, the Barefoot Bar being the most popular. It's in the heart of the village right on the main road. There are a couple of others on the beach. And there was another one in the village, the Purple Space Monkey, but it closed and was being renovated into something else, maybe another bar, maybe not. If you happen to be in Placencia during Lobsterfest, though, the village goes kind of crazy all day and all night. But that's in late June.
Getting there I guess you fly into Belize City and take a hopper down to Placencia right?
Right. You could rent a car and drive there in about three hours, but most folks opt for the 30 minute flight out of Belize City. There are two airlines, Tropic and Maya. Some people, e.g., Peter above, swear by one or the other, but in my limited experience I've found them to be pretty much the same. The flight is a very pleasant and quick way to see the Belizean coast. If you do opt to drive, the Western Hwy out of town is nothing special but once you get to Belmopan and take a left on to the Hummingbird Hwy then the drive through the Maya Mountains is fantastic.
Is the diving viz just as good if not better than AC or even Cozumel?
I've been to Coz only once, and the visibility was fantastic on that trip. In my two trips to Placencia as well as my three weeks on the Belize Atoll I've found the viz to range from excellent to very good, sometimes on the same dive. On one day during my first Placencia trip the viz was less than the length of my arm (look ma--no hands!) but that day was a couple of deep dives I was doing for a PADI Deep Dive class; otherwise we would have dived elsewhere with better viz.