My Clio is fairly accurate while the car is running, although when it is stopped (ie. the few occasions I've popped out and turned the ignition on to check it), it tends to under read.
In the Focus, full->half seems to last forever, then half->empty seems to disappear in about 20 miles.
Digital read outs in litres are no better -they are still getting their information from the same sensors. For example in the VX220, in the space of about 1 mile, it could read anywhere from 11 - 21litres.