I don't know if cecilia is still around on mumsnet but if so she'll be very amused at my replying to this (sorry, private joke).
I started a job share 2 days a week 3 months ago. I still do my old job the other 3 days a week where I have an assistant instead of a job share so I see both sides of the coin. It's not a secretarial job but I think the issues of communication to the people you are working with apply across the board.
Personally, I think the organisation gets considerably more than 5 days work a week out of having 2 of us. We're rarely away in the same week so there are no long periods of absence. I work the end of the week in the job share and I am considerably fresher coming into the new challenge than I would be at a full time post by Thursday.
It really does take time to get your head round it. It's very easy to try and do it all but you do have to work hard to set yourself some boundaries. It's also hard to set enough time aside to write all the important things down/speak on the phone, but that really helps.
My experience is that you also need both partners to be open by nature, good at sharing, and happy to admit to anyone that you don't have all the answers but will do your best. I am full of admiration for my jobshare partner, and she is a huge pleasure to work with. I know she does some parts of the job better than me, but I can live with that (I hope she can!). But of course that means our employer gets more skills from us jointly than they might with just one person.