Bought this a couple of years ago. Haven't used it that much. Still in very good condition with all the instructions, soft case etc.
Works out your elevations/windages based on data you enter into it and taking into account environmentals it measures - wind speed, temperature, humidity etc.
As long as the data you put in (bullet BC, weight, MV etc.) is good, it can be very accurate at predicting what you need to set on your scope turrets.
These are £400 new. I'm selling it for £215 including postage anywhere in the UK.