Something I learned long ago is that you announce BEFORE YOU START PLAYING what time you're going to be leaving the table.
There's nothing worse than holding everyone elses' money and saying, "well, I guess it's time for me to leave."
Mark
Nothing is worse than the case you state, but what annoys me even more is players who get up a good amount and then stop playing. They throw away solid hands and sacrafice their blinds just so they don't lose what they have won.