Impressive start, and could directly compete with Voila, if only it would capture video the same way it does stills (with selectable box tool), and capture the computer audio (e.g. your Skype video conference call's sound, and whatever you have iTunes playing quietly in the background at the same time, or whatever). Basically, I'm going to end up paying for Voila, unfortunately, because Monosnap just can't handle this core functionality yet.
It will capture fullscreen, or a big vague area of the screen that can be resized down a bit before recording, and it with include audio-IN, e.g. from the mic, or an external music source, so it's probably good for narrating app demos or something. If you want to do your own videogaming vlog on YouTube, this may be the free app for you. :-) If you want to record a specific corner of an existing video file with its audio, for example, it is not the app for you. Nor if you want to rip, to a new video file, some Flash video from a website that is resistant to the usual Flash downloading plugins.
I also agree with other comments about the "nagware" aspect of this. Many of us do not need, use, or even trust cloud services like the one being promoted by this app. It's not that it's unreasonable that this app is ad-funded, but there needs to be a way to say "I really did mean NO."
That said, it's great for still-image screenshots. If that's what you mostly want, this is vast improvement over OS X's built-in screencapping.