Let me just give my actual experience. We used Fairy Godmothers service, set up through the Concierge at the GF. I have to say that although the experience was not BAD per se, I would definitely make my own reservations, probably with a different service. Here's my story:
We asked Concierge at the GF to arrange a sitter for us, and they did so through Fairy Godmothers. When the time came, I was a little surprised at the sitter. I expected her to be a mother or grandmother, as it says in the Unofficial Guide, but she was actually fairly youngnot long out of college, dressed fairly trendy. I didn't ask her if she had kids (I forgot), but based on her age, and the fact that she was planning to fly to Phoenix for work the next week, I doubt she did. I asked how long she'd been working for the company, and she said about 6 months. When I asked her how she started working for Fairy Godmothers, she said, Oh, a friend of mine in college worked for them, and I wanted some extra money on a flexible schedule, so this fit perfectly. Nothing about liking kids or anything like that, just that it was extra money and convenient. I also expected her to come with games (Gertrude at FGM said she would), but she had none (thank goodness my DH and I had bought Jamie a new set of Buzz Lightyear figures that we'd saved for that night!). I at least felt confident that she was a kind-hearted personshe worked for the ASPCA, and was currently working on reuniting pets found after Hurricane Katrina with their owners (the trip to Phoenix was to deliver a cat they had matched with its owner!). We left Jamie with her as he ate Pizza (his favorite food) from Gasprillas, and he didn't act at all unhappy to be left with a sitter. (The pizza no doubt helped!)
We arrived back at the room much earlier than anticipated (11pm), barely meeting the four hour minimum for the babysitting. To my dismay, my son was still awake! That right there told me that the young lady didn't have much experience with kids, since she obviously didn't want to be authoritative enough to make him go to sleep at a decent hour. The price was only $64 for the four hours, plus I gave her a $21 tip (maybe too much, but I figured 20% was expected, even though she didn't live up to my expectations). Next time, I'm definitely going to try Kids Night Out, and I'm definitely going to call and ask a LOT of questions. Concierge served us well in almost every instance on this trip, but this particular instance, I think I would have been better doing it myself.
So my advice is to definitely do your research and ask a LOT of questions, and then make sure you get what you ask for when the sitter arrives.