garmin gps?

[t-maxxracer32]

hey guys i saw one of these at circuit city for 150 bucks and ive seen other people use these to see how fast the car can go.

how do they work?
whats the max speed it can rate?
what kind of things will it do with speed? (top speed... lowest speed... avg speed)
is this the right one?
http://www.circuitcity.com/ssm/Custo...etailReview.do
i cn get a actual radar thing for 80 bucks... would that be better than this?
can the e-tex gps do anything other than speed for rc people?



will probably be buying it today because i wanna see how fast my revo is!

thanks for any help!