Ok, I will speak from experience - I am a working (part-time) mom, my kids (2yoDD and 6mo DS) have had a Nanny from the time DD was 4 mos old (same one). We also have a steady saturday night sitter - EVERY saturday night.
Why? Because I believe that a well rested child is a happy child. My kids go to sleep between 7 - 7:45 EVERY night. So there are people (and I am NOT saying this is anyone here) who would rather drag their crying, sniffling, or sleeping kids through the parks so they can catch a parade or see fireworks, than let them take their baths, read their stories, and horror of horrors, have a "playdate" with a new friend, and settle into bed at a reasonable hour. Which is worse, an overtired, cranky toddler in the happiest place on earth, or a parent choosing to abide by a schedule and let the kids go to sleep when they should? also, my DD is a very light sleeper and I couldn't have a light on or the TV or she'd never go to sleep (believe me, we've tried it!)
I have used sitters in the carribean, at a Hyatt resort, and in a resort in Canada - some of the sitters have been college kids, one was a pre-school teacher looking for some extra cash, some have been hotel employees. We have a routine - I meet the sitter before my kids do, to get a sense of them, then I have them come about an hour before we leave so we can help bathe the kids and get them ready for bed. We let the sitters read their good night stories, and we remind my daughter (DS is too little to care, and he's happy to have his big sister around) that if she needs anything or wakes up, so and so will be right there - most of the time, she has a great time manipulating the babysitter into one more story, water, potty (insert excuse here) just because she can.
And, EVERY SINGLE time, she has woken up in the morning and asked where the sitter is and can she play with them again.
We have had NOTHING but extremely positive experiences - in fact, we were in Jamaica a few summers ago, and we used the same sitter every night - she came with us to the airport, and we even send birthday and holiday cards, still!
Yes, the first time is tough - you worry a lot, and personally, I wouldn't leave the resort premises - this way I can sneak upstairs and check up if I am at all worried, OR the sitter can find me if for any reason my daughter or son seems to need me.
But it is important for everyone to have a vacation - my kids spend their BEST time with us. And they don't care if I go to have some dinner while they are sleeping - as long as I bring her back a treat (LOL!).
So while some of you might think I am a heartless, selfish parent, I think I am more selfless be spending the money I might havev spent on crap to pay a sitter so my kids can get their 12 hours of sleep!