Largest agency in the country by a good bit. Don't look at just one agency, get involved with several until you find some good fits for you. If you are looking at just one particular assignment, that goes double. American Mobile tends to low ball new travelers to facilities such as Stanford where they have an exclusive contract. Shop around is the rule, as AM does use other agencies to staff Stanford (for example) as well. Yes, they will choose their internal travelers first, so if it is a must do now you are best off with them, but usually (and paradoxically) their subcontracting agencies will pay more.
Large agencies are great for their large choice of assignments though. And often the best choice for a new traveler which I'm guessing you are.