In the early days of Lazarus Paul, Micha and I used IRC a lot to coordinate graphics development or the win32 development. At that time I was the European evening on line and Paul the Russian evening so we covered a pretty big part of a day.
In such case the questions asked got pretty quick answered.
When you are partially online, I prefer the forum, since you can give a response in your own time. Disadvantage is that you don't have an immediate answer. For what I'm coding now I sometimes wished I could contact the devels of some 3td party lib through IRC, but in most cases, some googling helped too.
Marc