An AP really suits us because of the flexibility, but the thing I underestimated was the hand holding. It is a LOT of organisation explaining what to do, where to go, how to get there, how to open a bank account, how to get the best phone deal, what to do when plans change, what to cook, how to cook, how to book bus / train travel in advance to get cheaper tickets, etc. etc. Our AP will do almost anything he is asked to do, and get it right most of the time, but almost never does anything on his own initiative. The previous one was similar (more initiative but not so good in other ways).
They key for me is not so much the money (though obviously if you can't afford a nanny or CM you can't), but what do you need? If you need regular hours, childcare only, each week, then you don't need an AP. If you need some mornings, but not all, different each week, some afternoons (ditto), some schlepping about to activities, some cooking, some playing footy in the garden, some babysitting and someone around so the dog's not lonely then an AP is ideal. You just need to make sure you count up what you're expecting and not asking too much.
The other thing I underestimated was the amount of food another adult needs (3 meals a day every day). Also, even if you, like me, eat toast, or a banana, or nothing, sometimes for lunch, s/he may want to cook a plate of pasta with meat and veg (or whatever), so you need to allow more than you think for that!