Hi Stuart
I have that exact unit.
http://www.gunson.co.uk/item.aspx?cat=698&item=1835
Not a professional grade unit by any stretch of the imagination it is lightly built and not designed for any sort of rough handling. But is perfect for the amount of use we need it for.
I found mine easy to use and accurate.
As for the unit it's self.
The only complaint I have is the 2 sided foam tape holding the pump (the part sticking up) on to the unit is rather lacking but that is easily rectified with some better quality foam tape.
It does require a separate power source as the cord is not any where near long enough to reach from the back to the front of a vehicle. Which is fine as I have separate Automotive 12V Gel cell I use for that and the timing light.
Alternatively a person could just pick up a 12V Emergency Light Battery. Such as this one
http://www.emergencylightbatteries.c...A_Battery.html
This is a much cheaper solution to buying a spare car battery.
I set mine on the ground near the back of the vehicle so the moisture will drain properly.
Be sure to keep the unit it's self out of the exhaust when calibrating or you will never get an accurate reading.
The display is easily readable from the front of the vehicle even in sunlight.
The response is time on the unit it's self is fairly quick but not instant.
Make your adjustment and wait for the unit to stabilize. It will take about 10 seconds or so from making the adjustment to it showing up on the read out. So just go slow and be patient.
Really this unit is the only affordable solution to getting a CO % from the Tail pipe.
For what is it I have no complaints and it does the job it's supposed to.
Matthew