I got my invite in the first wave and pretty much immediately ditched the service. I just didn't see why I'd use it. I am a light Facebook user and I don't Tweet.
Recently I decided to give G+ another try and I love what I am seeing. First of all, the site is a pleasure to use and the mobile app is by far best in class. Second, I think G+ is absolutely the best social network for those who are interested in following people outside of their friends. Celebrities on Facebook have something to sell, and on Twitter there's not enough room, but on G+ I get long, interesting posts from famous people around the world.
I only have a few friends on G+ but I am starting to check it more and more and I can say without a doubt that if it ever hits critical mass I'll be the first to drop Facebook. Google simply has the latecomer's advantage, they got to watch Facebook stumble and build around those obstacles at their own pace.
Also, Google+ sign-in is a great developer product. The deep-linking concept is a very interesting implementation of sharing for mobile apps that borrows from Android's intent system.