I used Geoff's, and I had a pink cabin top - love his stuff. I went to the trouble of glassing it in and filling the transitions all around the edges to make it look good so I didn't need a headliner (see pics). The OHC fit really well with very little cutting/fitting; most of the work for me was making the transition look good around the edges, which you wouldn't need to do if you are using a headliner.

I put a lot of lights in mine, and love them when I need them, but don't use them very often - mainly when I'm starting up in the dark for an early departure, and the occasional night flight. They are all LED (of course) and include some simple "dome" lights for overall cabin lighting, red map lights for the front seats, and indivdual "eyeball" reading lights for all four seats. All are dimmable. I used the cheap black eyeball reading lights but I think I would use the fancy ones with the dimmer on the light (Sean Strasburg used these) if I did it again.
I used the Stein air vents (great) AND I added Geoff's NACA vent controller because you just can't get all the leaks out of it and it's really nice to be able to shut off all air to the OHC in the winter.
You will need to figure out how to butt the OHC up against the aft bulkhead (top half) so that it seals, but I just did it with a little piece of rubber seal just like the gap cover seal that goes under the HS.
One other thing. The OHC is great for running wires as well. I used the tube over the glareshield to run cables to the three GPS antennas I have up there: two external and one internal to the console. I also ran the power/ground wires for the LED lights. Without an OHC it would be real hard to put antennae on the cabin top.
My $0.02