I second everything nick said, many people will knock dynos in favour of on road logging but unless you have first hand experience of datalogging and mapping on a good eddy current dyno you cant really comment on how one method is better than the other.
I.e you want to map/check a data area for boost, ignition timing and fuel. This data area is at 65% throttle at 6000 rpm. You go out on the road with one person driving and one checking the data and correcting the map and as you hit 6000rpm at 65% throttle you have a fraction of a second to look at the 3 required parameters and before you know it you are at 7000rpm.
On a dyno you can hold the car at an exact rpm amount and it wont allow it to accelerate further so you can analyse the data, correct it, move onto the next data area etc... then at the end of this process you carry out a sweep (dyno run) through the modified data area and compare the power output and emmissions to see if the alterations have improved the cars performance.