pretty much everyones feelings until you start working with it daily. It takes a few months/sectors for which ever flight planning software your using builds up its experience database. Then it is pretty much spot on.
Ours even gives a bracket of fuel depending if your late or early. Ie if you depart an hour late it wants you to have an extra 75kg on board because of historical ATC delays if your into a certain time frame on a TP that burns a ton an hour.
Normally it gives you a bit more than you really need due to shortened SIDs and Stars.
What it does mean though is those that take the piss with cost index and cruise power settings get found out pretty sharpish. Also dispatchers get a bit more conservative and realistic when planning. If your used to having loads on and the pilots taking extra then they shorten everything and the resultant fuel is a compromise. If you just fly what they give you and divert when required it only happens a couple of times before they get hammered by the accountants.
Its those that take the piss the most, with not playing the game with power settings that seem to shout the most about it and continue to shout the most about it. The rest of us just get on with it in the EU.