I haven't done it but it was one of the options we were considering for when I go back to work later this year - as we will have two in daycare so thought it might be more cost effective.
Short answer - it's not. We'd need to have probably 3-4 kids before it would be competitive with daycare.
Qualified nannies charge around $20 an hour - so if you work full time and need them there before you leave until you get home, say around 10 hours a day - that's $200 a day. So it can get rather expensive.
I think it would definitely be viable if you had a few kids under school age, or could share with another mum which is another possibility.