is an excellent way of gaining extra tips and resources for your department, as well improving your own marking skills. the money is ok, but not brilliant actually, and i do it mainly to get an insight into what examiners want. i'm lucky as i stop teaching in june(because of study leave) and don't start marking until july. i can imagine it would be harder for people working in schools..
what i hate most is the admin, sending papers off at the right time to the right place, but this year i'm marking online so i'll see how that goes...
Edexcel don't require you to have a pgce (but do require you to have completed a degree in the relevant subject) so it could also give you an edge on interviews etc if you have that extra experience. and it may also be an option for people on maternity leave as the work is done at home.
i'm not being paid to say this honest, i just want someone to complain with! i'm marking edexcel GCE module 6 (the synoptic paper this year) and i know they are still short of examiners for this paper.
here is the website where you can apply if you are interested.