We are doing just that and the results are not quite as I had imagined. We started in september and we have moved them on to their second strip a couple of weeks ago. We have a scrubby slopy field, lots of rocks with a fair smattering of thistles and bracken (Did lots of scything and bracken wacking over the summer) and the idea of a fresh start with a beautifully turned over and manured field (as shown in seymour (the complete book of self suffieciency - NOT a book I would recommend other than for the bit before you get your smallholding!!

was tempting. The results so far are interesting - the pigs (4 saddlebacks bought at 10 weeks) do plough the ground sort of - they seem to get their snouts just under the 'turf' and turn over a big flap, eat what ever it is they are after and move on. They neatly go AROUND thistles!!!!! Then there is their toiletting area (about a third of the strip) which they leave untouched and their water trough and house area which is mashed to a hard /soggy (depending on weather) pan as described above. It may be that if we kept them on the strip longer it would even out?
But what we are left with is a lumpy area with turned over 'divots'. It all could do with a good harrow before seeding - but guess what this field is too steep and rocky for that (hence its condition probably!). So I'm hoping the winter will do some of my work and then I will wake up one spring morning with 'the solution' (which wont involve me and a rake!) - I love it when that happens! Good luck! In short If your field is accessible for harrowing, I think the pigs will do some good work (but not all of it!).