You will find that many of your city's better recruitment agencies are recruiting for the same roles, therefore signing up with several agencies may be a waste of time and may in fact create a logistical nightmare for yourself in keeping track of which agency has sent your CV where.
In saying this, some firms will initially brief a selection of preferred agencies or one agency on their vacancies and then if those agencies do not provide suitable candidates within a set period of time (perhaps 2-4 weeks) the firm will then open the role up to a number of other agencies as well.
My advice is to ask your colleagues to recommend an agent - not just the name of an agency but the name of a consultant. Call those consultants and find one who you gel with. Ask that agent to represent you and give them some time to find you a role. If you feel after a few weeks (or a few months for more senior lawyers) that it is not moving quickly enough for you, then perhaps see another agency as well.
If you are using a number of agencies it is essential that you keep a good record of where your CV has gone as it can be rather embarrassing for yourself and your agents when a firm advises that they have already received your CV three times in three days!