As it happens there was a FreeBSD Vendor Summit this week and so there's a few decks of interesting slides from the event. Besides covering the FreeBSD Bhyve virtualization hypervisor, another interesting track was on LLVM's LLDB debugger.
FreeBSD has been bound to using an old version of the GNU GDB debugger (GDB 6.1.1 from 2004) that is still GPLv2 licensed before they went GPLv3, which is hands-off for the FreeBSD project, but besides the more liberal license they are also finding other advantages out of supporting LLDB.
The benefits seen by the FreeBSD developers include the speed of the LLVM LLDB debugger with being multi-threaded and relying upon performant LLVM classes, efficiency with a minimized memory footprint, and greater accuracy with improved abilities for breakpoints and expression parsing and breakpoints always being symbolic.
LLDB can be supported by Apple's XCode and there's also a command-line interface and a Python front-end. With LLDB's Python support there is a built-in Python interpreter for scripting and its support could be extended to other programming languages.
In terms of FreeBSD's LLDB support right now, 260 test-cases from their suite are running and only x86_64/amd64 support is in good shape with the i386 version not working and the MIPS and ARM support being a ways off. For 64-bit FreeBSD 9.2+ users, LLDB should work but not all of the functionality is working great -- especially for kernel debugging.
Some medium term plans for FreeBSD developers working on LLDB is 64-bit thread support for PTrace, Watchpoints, MIPS host and target support, and addressing LLDB test suite failures. Longer-term goals are to support ARM, kernel debugging, and remote debugging support.
The Linux support for LLDB is in a similar situation where the support and compatibility is not nearly as good as under OS X, but improvements are being made and there's non-Apple developers beginning to contribute upstream. Among the LLDB debugger contributors outside of Apple include Intel, FreeBSD, Debian, Valve, NetBSD, and others.
The FreeBSD Vendor Summit slides on LLDB along with more information can be found in PDF form via the FreeBSD Wiki.