Pros: Appearance can be highly customized, including size and button colors.
Cons: Animation doesn't work as expected. The seconds digit slides down from the top, stop, then disappears, then the next digit slides down, stops, and disappears. I think it would be better if the digits would slide in from the top continue to slide out at the bottom, with each digit following in sequence. Maybe it's too hard to get the timing right for this.
When used as a widget, the counter and widget title aren't enclosed in a div, so it's hard to style them as a single unit. For example, I wanted to enclose them in a shadowed box, but couldn't using the counter as a widget. I was able to get around it by pasting the shortcode into a text widget, but that seems rather kludgey.
Lastly, I don't know if this has to do with the theme I'm using, but the seconds digits always seem to wrap to the next line, no matter how wide the container is.