A reason for the fork was "slow progress", but a new stable version of Miranda IM was literally uploaded 3 weeks ago.
The reasoning still holds true if you read their rationale. Slow progress was not about new minor releases, but being able to move the app forward. Miranda IM doesn't even have complete Unicode support yet. Miranda IM's updates have been relatively minor. Miranda NG has already switched entirely to to Unicode and added support for adding/removing plugins while the app is running, which is pretty handy. Additionally, all their plugins are up to date, and they have a plugin updater that can run automatically to update them for you, all pretty handy.
It's not really a 'successor', that's what the devs call it and we just included that description.
The PAF package of Miranda NG includes the 6 languages they make available as well as another 15 that I updated from Miranda IM format that were reasonably complete. I've posted to the Miranda NG team that they're available so they can make them available to their users as well.
If you're a fan of Miranda IM, NG is worth trying. I'm not sure which version will 'win' if that will even happen, but we had lots of users request the NG fork so we made it available at PA.c. Personally, I'm a Pidgin user (and use the Pidgin Skype plugin to connect to a local Skype install when at home).