I have a Garmin 330 and it is a wonderful box. It does have some nice
features like a flight timer, readout of pressure altitude, as well as an
aural alert when leaving an altitude.

However, the most useful item is the Mode S, connected to a box that will
display traffic. Even on severe clear days it is amazing how much traffic is
out there that you will not see. Most of my travels in the Midwest have had
coverage (Chicago, Milwaukee, Indiana, Minneapolis), while some smaller
locations did not (South Bend, for example).

Regardless, for the extra few relative dollars over a regular transponder,
it was definitely worth the cost. It sure beats $20,000 for true TCAS or
TCAD.