I finally tried using the trackpad as a D-pad in platformers and I've got to say... it really isn't that great. It works, certainly, and works better than it probably should, but I had problems with it registering the wrong input in situations a real D-pad never would have. For instance, "down" registers constantly even when my thumb is only moving across the horizontal bar of the indent.
I'm not sure if the trackpad is just glitching or if it's the same problem that occurs when using the trackpads to aim- that any flesh of the thumb other than the very tip touching can skew your position. I found adjusting the D-pad dead zone to minimum helped, which is counter-intuitive since a larger zone should eliminate the accidental down presses.
It might just come down to the limitations the trackpad has in emulating the rocking action of a D-pad. On a real pad, these same motions register distinct inputs. Without an actual physical rocking action, the trackpad may not be able to register presses the same way. Knowing Valve, they'll probably have some genius improvement in the next two week that will address this, too.