How mature is the current Open Dylan implementation?

Well, the current implementation, Open Dylan, has been developed since '95 by Harlequin and Functional Objects, before it became open source in 2003.

How is maturity defined? The compiler includes an IDE (Win32 only, sorry), CORBA, OLE, all in all 850000 lines of code. This is (nearly) all Dylan code.

Meaning, unlike other languages, it is self-hosted. The compiler itself is written in Dylan (with an exception of some lines of C (~4000 lines of code) for the runtime/garbage collector binding. The back-ends are either native x86 code or C.

Please do not confuse this implementation with the two others, which are around: Gwydion Dylan (also written in Dylan; developed formerly at CMU, now open source as well), which compiles Dylan to C; and Apple Dylan (closed somewhere in Apples big storage ;).

According to a comment on Hacker News it isn't ready yet. They just started to revive the project.