I've heard of some of these, I think there are some field reports on PN.
I've not used them either, but I think you will find some of them to be legit. The thing is they charge a low base rate, but when you see the girls they will want you to pay for the extras, so £20 for OWO, £20 for kissing etc. The outcall ones, as said before, will charge for taxis etc too.
If you are after a basic service you could be OK. Otherwise you might end up spending more than a regular agency and the girls tend not to be quite as high class.
If you are after cheap you maybe better sticking to AW. Although if you do use them, make sure you write a review and let us know how it went.