The best thing (and speaking purely from my own experience) is you get to meet some really interesting people and no two days are the same. I used to work in an office 9-5 and that was the exact opposite - everyday was the same.
The worst thing for me is when you meet and teach a pupil with so much potential and in my case talent for Art - but they have a 'too cool for school' attitude and waste their chance, purely because they are either lazy, too complacent or as I say feel it's somehow uncool to do well at school - that's a real shame and more common than you might think - they usually come round - but unfortunately when it's too late.