i don't have any experience as such in them, but i do know there's hardly any listings on them and most people never look at them anyway. they may be cheap but thats because they've got virtually no hits on the website, and as they're national, you might get 2 or 3 properties in a 50 mile radius of where you want to be buying. your best bet would be to use a well known, local, trusted estate agent that does lots of advertising. most people use rightmove these days anyway and (i saw the other question you were in about agents fees) i've never seen a single property advertised that wasn't a major agent - and i must look on rightmove at least once a day for myself, and i'm on it aaaall day for work (i'm in property investments!).
i don't know anyone who has ever used one, but i do have a friend who set one up...and shut it down again 3 months later for lack of interest.
for the sake of the �50 or whatever it is, is massively cheaper than agents fees, but the chances of you finding a buyer are much much lower.
...at least IMHO!! :D