Neat, but $5? As an /introductory/ price? I don't have a Mac, but I can't imagine it's that troublesome to capture, annotate and share screenshots on one. I guess I'm just not the target market for this sort of thing.
But it has robust backend infrastructure and it has been around for a while.
You know... trivial extra feature vs. maturity and reliability... it's not a hard choice. Besides there are CloudApp clients that support annotations if that's what someone really needs.
This doesn't have annotations, but it's a free alternative: http://grabbox.devsoft.no/