A few months ago I convinced my team to start to use 37signals' Campfire to be able to communicate with each other, especially when someone was away from the office. It worked great for the first week, then after that people gradually started to 'forget' to log in, until in a few weeks time I was the only one logging in frequently. So I haven't even bothered to mention Yammer to anyone else on the team. I know they won't use it.

Yammer is just like any other collaboration tool. If you (or others) don't bother to use it every day, then it's virtually useless. But when used by everyone on a consistent basis, it can become an invaluable tool.