One problem you may find is that their website might not be up to date and some of the properties you have picked out will no longer be available. They will also try to keep hold of you once they get their claws into you and if you have appointments with other agents they will make sure they have extra (usually) totally unsuitable) properties to show you so you miss your appointment with other agents.
Be very firm. Don't let them mess you about and if they are showing your properties that do not meet you criteria tell them so and don't let them waste your time.
They will use any tactics they can think of to stop you going with any other agent.