Currently, badly. The FDM is yasim-based, but I will almost certainly have to rewrite it using JSBSim - yasim is just too limited to model what I need. It's good enough for a few screenshots though. I don't have any proper instruments either, I'm using Phi, the default HUD, and a crude status display in a canvas window to get an idea of what's happening.
Anyway, the idea is that the spacecraft can fly in 3 different modes, all of them aided by the flight computer: VTOL mode (controls work like in a heli, used for takeoffs, landing, and near-ground precision maneuvers), aero mode (controls work like in a supersonic aircraft, used for climbout, atmospheric reentry, and in-atmo cruising), and ballistic mode (used for anything out-of-atmo: second-stage launch burn, orbital maneuvering, deorbiting, early reentry; also interplanetary travel, but that's not going to work anyway). VTOL works fine, ballistic is mostly fine but suffers from gimballing problems in the auto-stabilizing code, aero is currently hopeless.
In terms of instrumentation, I want something that looks consistent with the things we see on screen, while still being practical.