I've successfully completed an in-place upgrade of a Ghost blog from 0.7.9 to 0.8.0. Looking at the release notes, the only braking changes seem to be in the theme API. So unless you're using it to build out a custom theme, upgrading shouldn't be an issue.

The team at Ghost wrote up a separate post about those theme API changes. To quote their summary:

1. All partial files MUST use the .hbs file extension.
2. Handlebars helpers provided to HTML attributes MUST be quoted, e.g. <div class="{{post_class}}">
3. ../ in handlebars now behaves naturally and doesn't require multiples when using nested if statements.

Check out the post for all the information and review their upgrade documentation.

Everything worked perfectly for me. No issues so far. I use custom theme, so I will need to modify it to support the new features of ghost 0.8.