Agree with Smiths: for London, agencies are overpriced (mostly).
Elsewhere: advantages of agency (mostly) easier to book, plan B is offered by them, generally young and pretty. If it goes wrong agency might try to make it right.
Disadvantage: Might not be able to talk to the actual girl, more have poor DFK, more don't offer OWO RO. Never heard of agency girl offering alevels, but I guess everything exists out there. So if you *must* have these services you need to still do research separatly (i.e. here).
Net: helpful when in a hurry/lazy/know a girl you trust. In some locations, it is a great choice.
So on the flip side, indy can sometimes offer better value for money, but there is greater risk. In some places the best indy girls charge the same as agency. I have no problem with this - they personally get more per hour, but same cost to me so good for them.
I end up using both (plus parties and brothels) - some pathetic drive to explore new girls I suppose.