From my experience (financial technology, very hard to find finance jobs in London without going through recruiters), the main qualities that a good recruiter has is that they're willing to take time to get to know you, they're willing to admit when they don't understand something in the spec they've got and will ask you about it rather than trying to bluf, and that they still take your calls and take time to talk to you when interviews they've set up haven't panned out.

A good recruiter is interested in a long term relationship - they know that people tend to change jobs every 18 months to 2 years (in my industry at least) and they want to be the first person you'll call each time you get itchy feet and because they know you, you're easy for them to sell to your next employer and they know what type of company you'll be happy working for.

The three best recruiter's I've used have all done one simple thing that got the relationship off to a good start - spending a solid hour or so sitting down and getting to know me over a coffee or drink within a week or two of first contacting me. This is a far cry from the recruiters who call me on an almost daily basis who 'chat' for five minutes and then I never hear from again.

I met both of them as recruiters who just happened to be working for the company I worked at.

I would say ask around. Chances are people you work with who have been placed, if you are at a good company, were placed by good recruiters.