I'd be more interested in knowing what agencies do have a good reputation / good developer experience along with the note to beware of the cowboys.
Hays (They're big but have been good to work with)
I'll add more as I think of them.
If you're looking for work in finance - you will almost certainly be going through a recruiter.
The best agencies? None. Don't use them if you can help it. If you have to the smaller the boutique ones tend to be better, if nothing else because they'll have the better clients and more interesting projects.
The killer is simply making sure that if you do the entire 6 month contract in 2 weeks that you still get paid for the rest or have a massive termination fee in the contract.
The agents don't care; they pass all costs on.
I also think that it is worth forming a good relationship with a particular agent. Meet in person if you can so they can see you are someone who seems competent. And if you ever have a friend that is looking, it does no harm to refer them to a good agent you know.