If you do, it will result in a partial migration with incomplete files with the potential for all sorts of headaches regarding permissions and corrupted system files. If you decide to interrupt the migration, it would be best to erase the entire disk and reinstall OS X. Then perform the migration again.
Consider using Setup Assistant, otherwise you will need to create a redundant account.
thanks for the quick response, I think I'll just try and wait it out
Often in Mac migration, it can be migrating a large file and give a false 'time remaining' calculation. This can fall quite rapidly at times. I have seen copy times drop from 20 hours to 30 minutes in the space of 5 minutes. It has never been a true indication of time, so don't panic too much. If you interupt it and do a full erase and reinstall, then you will be spending a bit of time at the Mac anyway. By advice, go to the pub and let it go. lol