I've replied to a few of these ads in local press, Craigslist, Vivastreet, etc. The majority will sooner or later take you to Adult Friend Finder or one of its spin offs. There are a few, mainly EE, girls looking for a way into the UK, and there are a very few that are genuine.
I had 3 meets during the past 5 years through these type of ads - this includes placing my own ad which, again, attracts mostly touts for AFF. None of those three were good lookers but there was sex on the first meet with all of them and in one case some repeat meets for MMF threesome with a mate of mine (something the girl was really up for).
It really depends if you can be assed to put up with all the dross in order to strike lucky. On balance, its probably not worth it unless this type of meet turns you on more than paying the going rate for the girl of your choice on the day that you want it.