I have one and love it.

http://img.photobucket.com/albums/v3...2/DSC02420.jpg

Shows you all sorts of stuff like trans temp, fuel trims, MAF (in various units), MAP (in various units), intake air temp, coolant, shift times, converter slip, etc the list goes on. You can't change anything like tuning it. But, it does offer two-way communication by commanding the fans to turn on at any time, CASE learns, fuel trim resets, stuff like that.