My few cents -
I returned from ML Sept 2010 and went from FT class teacher to 2 days a week doing PPA cover in a primary school.
Pros of PT work:
No parents evening/reports to write
No assessment data to hand in and justify in pupil progress
I get 5 days a week at home 
Cons:
No choice as to what I teach (whatever the FT teacher can't be bothered with so usually the 'dry' aspects of the curriculum)
No continuity of who I'm teaching, seems as soon as I get to know a class I'm moved to a different year group
No data means massive weighting is put on lesson observations, last one I had I was told to teach a specific area within a subject to a class I'd not taught before. Brilliant.
No chance of career progression (TLR/threshold etc)
No CPD - I lead 2 subjects, mentor an NQT and a volunteer who is training to become a TA but have been refused all requests for courses/training for these
Previous experience within the school is ignored (including CP issues - where I'd taught siblings so raised a concern)
Expected to attend all INSET days/meetings regardless of whether I work on that particular day (was initially expected to attend without pay)
No time given for extra responsibilities as 'you're hardly ever here!'
Still have to plan/mark/assess/make resources etc etc but marking can't be taken home unless on paper/in a special ppa cover book, resources hard to make without laminating equipment at home etc
Very little time to get the above done - This is my day
earliest I can drop at nursery is 7.45 - first in
arrive at work about 8.15 depending on traffic,
quickly set up (find out everything I'd planned for has changed),
out on morning duty at 8.40,
school starts 8.50,
teach until break,
break duty
Teach until lunch
20 min lunch (sit down for first time since getting out the car at 8.15
)
Lunch club
Ten minutes before end of lunch rush to a different year group, set up
Teach until home time
Staff meeting for an hour (and a bit!)
Meeting with FT teachers to feed back/check for next day
30 minutes to get everything sorted before I need to leave to pick up from Nursery
Arrive a nursery with 2 minutes to spare - last one out
I know all WOHP have to juggle their lives etc and I am lucky that I get plenty of time at home (ironically we do the least during half term etc as everywhere's so busy!) so I'm not whinging about being busy, just giving the example of what going PT means for me.
Please excuse typo's - have a child with a cold on my arm 