Got one mounted in my DD and I like it alot. Havent had to use the scantool portion of it but I like it for the instant fuel economy readings.
It also reads all the output from all the sensors so if your car didnt come with a particular gauge which you like to monitor (temp in my case) you can be monitoring that.
If you dont have an ODBII scan tool at all its definitely worth it for the extra features.
I already had a scanner for code reading but I still like the tool for all the other features. If you do alot os distance driving its amazing how much having the data in front of you will help you modify your habits for better mileage.
Mounting greatly varies based on the car. I made a custom mount for mine, took about 10 minutes.