Barometric altimeters are almost useless. Any time the baro pressure changes, you have to recalibrate the gauge by setting it to a known altitude. Most do a pretty good job of self adjusting for temp changes. You have to recal at least daily. Once this is done, they are pretty accurate. GPS alitmeter readings are the least accurate of the lat/long/alt measurements. When calibrated, a baro altimeter may be more accurate. I had no luck calibrating a baro altimeter off a GPS alitmeter.

THe altitude data is nice to have, but given the above, perhaps a mani vac gauge would be more useful overall, primarily as a fuel econonmy gauge.

A true magnetic vertical card compass from an aircraft application would be nice, too. I've never been wild about the electronic flux gate compasses in auto applications. The resolution can be as low as 45*. It's often useful to know if you are closer to nnw or wnw when all the compass says is nw...