The helix is coming along very well; the subroadbed is complete and once another 30 feet of track is laid, it will be complete.
Some progress pictures follow.
Test fitting the sub-roadbed.
This is a great overview picture, showing how the helix fits into the space and the first circle and a half build. Also in the picture are some #8 Atlas RH turnouts that I modified to give a slight curve to; works pretty well I'd say.
Status as of today; the sub roadbed is complete and the track has just a bit more that half a circle to go.
I purchased a custom helix kit from Ashlin designs; I specified a 34" and 36" radius for my helix. The cost was just a little more than the stock designs offered for HO scale, but I felt that my average rolling stock would look pretty silly and have some performance issues, and with the thought of having intermodal, autoracks and Amtrak Superliners running up and down the helix, I figured I was better off going large to start with and not having to rip it out later.