I do it for Edexcel GCSE RE.
The money isn't fantastic and it is hard work.
The exam I mark is all done on line, and it kills the eyes. If I was still teaching, I wouldn't be doing it. It was one thing marking actual physical scripts, which you could peer at if a meaning wasn't clear; that you could take to school, lock your door and mark in a free; and as long as everything was marked on time, you didn't get hassled. You also went to London for standardisation, so you could pick holes in the mark scheme and ask for clarification from others around, and generally feel more secure in what you were doing.
Now: standardisation is online - the mark scheme is as it is, and you have to try and apply it. You get to practice the mark scheme; but then you have to qualify (pass the standardisation questions) to be able to mark.
If you fail on one of the questions, then you lose your bonus payment (this is not made clear) which is also your incentive payment for marking on time. Bang goes your will to get things finished.
Moreover, if you haven't marked as many as think you should have done, you get telephone calls about it. For instance, the subject I mark has two papers; most students do the first for a half GCSE, some do both for a full. These papers are sat within a week of each other. The timeframe for standardising and marking the second paper begins before the deadline for the first paper being completed. I was given an extra allocation of marking for the first paper that was not in my contract; I kept getting calls saying, you haven't marked 'x'. I said I wasn't contracted for it,and wasn't marking it until I had a written assurance I would be paid. That arrived a day later on email. I then got a call saying you are behind on 'x'..... even though I'd only just got the contract to mark it!
Same sort of thing with the second paper; you haven't started marking paper 'b'. I'm finishing 'a'. But you're behind....and?
I do it to keep my hand in (cpd) if you like, as I am currently abroad and not teaching. It also pays the fees for my MA. I can fit it in as I am not working, but when I was teaching, I came home, did planning and marking for school, and THEN sat down to do the exam marking. Bed before 0230 is a rarity when you are trying to mark GCSEs after school and on top of your normal workload.
Plus points - it informs your teaching no end; you can teach exam technique much more effectively as you know what you are looking for as an examiner; and when you tell your students that the examiners are looking for certain things, they believe you because you are one.
Money wise for the exam session in June, which consists of money for having a broadband connection; standardisation fees; and the actual marking and bonuses (although I didn't get one of the bonuses) was £1482.26 net. However, that was 6 weeks work. I get more as I can reclaim the tax as I am a SAHM at the moment.
You could do it for a couple of years, but I'm not sure I will do it from 2011 as the new syllabuses will be being examined them and I haven't taught them. However, ds's school doesn't teach RE, so he may end doing an extra curricular GCSE RE so that I can say I've taught the new syllabus and have a student's exam result to prove it!
Hope that helps.