Temporary recruiters are not the same as executive recruiters. Temp recruiters do care about your day-to-day happiness and your career goals, but they focus mainly on their clients' short-term needs, not on your long-term wants. It's great that you want to develop your copywriting skills, for example, but the temp recruiter is not going to spend her day finding ad agencies for you if she's got fifteen regular clients asking her to find receptionists. If you know what you want to do, perhaps you should talk to an executive recruiter who can take the time to find an entry-level position in your chosen field.
Temping is not a guaranteed way to find the right job. It's tempting (no pun intended) to jump at the first job offer you receive from a company that you're temping for, even if you're not completely happy. You'll feel so good that your superiors have noticed your talents, and you may believe that no other company will appreciate you in the same way. But evaluate your reasons before you accept, to make sure that you're not falling into the trap of taking the job because it's comfortable or because you feel pressure from friends or family to "get a real job."
You may not get to work a 40-hour week. Depending on your skill set, your location, and the agency you work for, you may not be needed every single day. If you need to get a full-time sized paycheck, make it clear to the recruiter that you're willing to take any assignment over no assignment.
Assignments may disappear without warning. You may think you're set for a few months with a great position, but one day you could get a call that tells you not to report to duty. Companies often use temporary help so that they can easily drop workers if a big order gets cancelled or a product launch gets pushed back.
Companies don't necessarily give you the same perks as a regular employee. Even if your colleagues see you as a regular employee, upper management may not. You may not get to go to the holiday party, play on the softball team or be eligible for employee of the month. It can be quite demotivating to miss the fun office camaraderie on a "job description technicality."
The health insurance, if offered at all, isn't stellar. A lot of temp agencies do offer health insurance, but for the most part it's designed to "hold over" workers until they can get benefits from a permanent job. If you're thinking you want to temp long-term, you may consider supplementing this health insurance with another plan, or asking your parents or spouse to put you on their plans.
You're not always treated with the respect you deserve. Because co-workers know you're short-term, they may leave you out of the loop on long-term decisions, or forget to invite you to important meetings. Also, it takes awhile for people to get to know you and respect you. If you don't spend a lot of time at one place, you may not get the opportunity to earn that glorious freedom that comes when your superiors and coworkers trust you.
Temping may get in the way of finding your passion. If you don't know what you want to do, temping may provide a nice testing ground and you may fall into something you love. But if you have a career goal in mind, it may make more sense to spend your time researching and making contacts in that field.
In general, if you temp because you need a way to pay the bills while you work on another part of your life--writing a novel, planning a trip around the world, getting ready to move--then you'll probably be happy with nearly any situation you're given. However, if you look at temping as a guaranteed way to get your foot in the door at the perfect company, or if you have an inflexible budget, you might want to try other avenues to reach your career and financial goals.