The people at the Roundcube project have been busy, they have released an updated which should fix a lot of bugs according to their statements:
This update release fixes lots of bugs and improves the file upload when composing a mail message. Also some of the plugins have been improved and translated into more languages.
I’ve been using 0.3 for some time now and I haven’t encountered any of the bugs, I would almost say it’s feels like a proper solid release and not a 0.x version. If you want to know what’s updated have a look at the changelog.