This doesn't just alert you anytime someone mentions your brand (like a Google alerts style service might) - this analyses tweets to identify mentions that you might want to act on, like tech support issues, feature requests and (y favourite) when a journalist mentions you.
But that too already exists... I wanted to built exactly this ~6 months ago, but I didn't because there were already so many competitors back then.
Just because a service exists does not mean that someone else shouldn't launch something better. I love the simplicity of the service (reminds me of buffer that does one thing really well).