IO540
On the TCAS subject, I do most of my flying in the 2500-4000'
range and find Skywatch valuable.
While there are some non-transponder equiped aircraft
about, there are less than you would think.
The main advantage is making the encounter with other
aircraft less of a surprise and alterting you to how many
potential contacts you never saw previously.
I would say that on typical 1 hour trips it has reduced
the number of close encounters by a factor of 80-90%.
-- Andrew