I did some research on this, and from what I've been able to tell, it's just Garmin units that have a limit of 99 (kt's or MPH, I don't remember which). They do that just so that they can sell you on the aircraft version.
The other units are useful for 2 reasons: You get your true ground speed, and you get real bearing information. Those are both useful from the standpoint of having a backup, and being able to get to where you are going! That said, I have the Garmin III, which I love, and it was only $700. -Tom
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