Hello Bar is invitation based till now but If you don’t have any referer don’t hesitate to sign up directly. You may get access to your account in 3hrs. It depends on how much traffic they have.
These bars are now widely used and most bloggers said they have increased user engagement after installing the same. The more the site engagement means more traffic and revenue also.
Hello bar can be customized directly from its website and you are not required to edit codes inserted to your website. You can also flash latest tweet if you wanted to do so. Their are a lot of fabulous customization to change look and appearance.
Although, This is may be very easy to add to your blog but if you have well running site it may be substituted by your own codes. I have seen many people using their own custom codes to flash message on the header. Some best example are John Chow and Shoutmeloud.com. These sites use their own codes due to SEO and page speed.
You should try yourself at-least once and add this to your blog and feel the difference.