I just brought my plane back from annual. Apparently the shop forgot to
put the transponder antenna back on. I was impressed with how easy ATC
made flying IFR w/o the transponder. They did not require me to give
the full non-radar reports. They just asked me to verify my altitude
and report certain intersections. Some of the time they were able to
radar ID me based on my primary target but other times they claim they
didn't see anything.
I had to fly IFR once before w/o the mode C (it broke in flight) but
this was the first time w/o any trasponder.

-Robert