1. The temp agency takes a big chunk out of your paycheck.
Your employer is actually the temp agency, not the place where you've been assigned. The agency charges the other business a fee, and then the agency pays you. No doubt the amount you get is based on the fee the business was charged, but you never see that number. You tell the agency what the minimum salary you'll accept is.
Short-term temp jobs do usually pay less than the same work in a permanent position would pay, but it's not abysmal. The national average in the US is $12/hour, which is enough to live on even here in Seattle, one of the most expensive cites in the country. (If you have a couple years of experience, there's no need to set your minimum any lower than the agency's average.)
2. Temps get treated like dirt.
Every job you get will be different. Most of my experiences have been positive. And while I'm sure many temps have worked in places that were downright mean, I think most negative places are just cold -- in other words, nobody talks to you or looks at you. You can let this bother you, but why waste the energy? You're out of there in a couple weeks anyway. It's possible the employees are too snooty to talk to the lowly temp, but it's just as likely they are too busy and stressed out. Remember that in situations like these, people are still very grateful that you showed up. At my last permanent job, we had to bring in a temp for a week while our receptionist was on vacation. I had no time to talk to him or make friends with him, but just having someone there to answer the phones kept me sane.
But, as I said, every job is different. In many places I've been treated like family, and those jobs more than make up for the colder ones.
3. You are the temp agency's slave! They will call you at 7:30 in the morning and order you to go to whatever shitty assignment they have for you that day.
In reality, your staffing coordinator asks you at the interview what kinds of jobs you're willing to do and are interested in. When something becomes available, they call you and ask you if you'd like to take it. You are always free to say no. And they usually give you a few days' notice, although you will earn brownie points if you come through for them in a last-minute pinch.
4. Temps get handed all the shit jobs no one else wants to do.
...this one's true! It just helps to be easy-going. See "You are willing to do anything" on the previous page. (In my experience, doing menial tasks is much less bothersome when it's not a permanent position, since it doesn't get in the way of your regular job description and you don't have that grating feeling in the back of your mind that your boss of two years isn't respecting you for what you're worth...... er, anyway.)
-Temps are free agents. They have no boss, only clients.
-Temps take time off whenever they want.
-Temps take and reject whatever assignments they want.
-Temps amass new useful skills at an astonishing rate, making each job better than the last.
-Temps get to work with a new group of people every few weeks, giving them the opportunity to meet lots of cool people and make more friends.
-Temps get to see up-close several different industries they may never have even thought about before.
-Successful full-time temps are frequently told how awesome they are.
-Really successful full-time temps are in constant demand by different agencies, further reinforcing the knowledge that they are awesome.
-Imagine you're a temp, and you accepted an assignment that turned out to be awful. The coworkers are mean. The work is tedious. The money isn't worth it. But guess what? It's OVER before you know it! Temps aren't tied down to any one lousy job.