After a long wait, Mantle 2.0 is finally here. I got the opportunity to write most of the additions and learned a lot from Justin's insightful code reviews.
For those that upgrade, it brings rock-solid error handling and makes it easier to extend adapters to handle reoccurring transformations. Check out the whole Changelog for the nitty-gritty. I've been using it in production for a while, and it being so solid was probably the main reason why the milestone was stalled at 95% completion for so long.
Sadly, I don't think we'll ever see a Mantle 3.0., now that Swift shows us a clear way forward in terms of value types and immutability that is not really compatible with Mantle. It's fundamentally an Objective-C project.
That being said, you should definitely have a look at my buddy Felix' Pistachio framework, it shares many of Mantle's ideals and is easy to mix with specific serialization formats, like Argo for JSON.
I hope you find Mantle useful and if you use it production, I'd be happy to hear from you.