It is similar to things you were asked in 'Total time spent on completed tasks' topic, but.

It would be nice to have additional field "spent time". When I click complete on task with filled 'estimated time' field it should ask me how much time I actually spent. Then a lot of nice features can be added: total time spent last week, correctness of my forecasts (estimated/spent ratio + chart), charts of spent time, e.t.c.