I do the Overground_Jubilee line commute every day, leaving in the morning any time from 7.30-8am.
Re; getting a seat....forget it! Your best chance is on the London Bridge train but that isn't guaranteed.
As previously mentioned the Overground is air-conditioned so it is'nt too hideously hot in the summer months.
You wont get a seat on the Jubilee line either so you just have to resign yourself to standing all the way to work.
The worst thing about the commute is the hideous scramble and bottle-neck trying to get onto the escalator at Canada water to change to the Jubilee line.
These negatives apart I am very delighted to have moved to Forest Hill and its well worth the 45 mins discomfort in the morning!