I'd tell them to know what they are getting into. Here are some negatives and positives:

Negatives:
-Low salaries relative to your education. Teachers don't get paid very well compared to others with comparable education. It's generally enough to live on, but, like someone else said, you won't get rich. Salaries can vary greatly from district to district, and even within districts, depending upon education and experience. I live in the Bay Area in California, and the salaries for a first year teacher range from 40 to 60K a year, which is not bad, but everything is more expensive around here (1/2 a million for a condo!). Other places that have cheaper living expenses have lower salaries.

-Time. You work a lot. Most people seem to think that teachers are lazy because we get summers off, as well as breaks during the year (winter, spring). First of all, I don't know a single teacher who doesn't do at least some work during breaks, and many work at grading a good part of them. During the summer, most teachers are planning for the upcoming year, tweaking units, taking classes, or working summer school. During a normal week, I probably put in between 60 and 75 hours of work (at times more) depending upon what kind of work is coming in to grade (I teach High School English). I calculated it out once, and we work at least as many hours per year as somebody who has a 40 hour a week job does. You will put in a lot of time, and it does take over your life a bit.

-Respect. A lot of people do not respect those who teach. They believe the old Shaw quote "He who can, does. He who cannot, teaches." In general, the people who believe this the most are the ones who would be eaten alive by a roomful of 30 15 year olds. Most of them couldn't get the to stop talking, let alone actually teach them anything. I've had the distinct pleasure of seeing a person who has disparaged teachers go into teaching and realize that it's not as easy as it looks.

-It requires faith. This could be either a positive or a negative. You hear a lot how teachers change the lives of the children that they teach. This is true, but most of the times the teacher will never know (or at least won't know until later) the impact that they've had upon a kid. You have to have faith that you've done a good job, and hold on to the kids who let you know the impact that you've had.

Positives:
-The kids. You have to like the age group that you teach. I could never teach Junior Highers. They have too many hormones that are going crazy, they are no longer at the stage when they think that teachers are cool, and they (for the most part) don't have the intellectual capacity for higher-level conversations. Some people love them, and more power to them. I teach High School because I relate well to the kids, and because they can actually speak intelligently about complex ideas.

-Your colleagues. By and large, teachers are interesting, fun people with diverse backgrounds. I've had a lot of fun getting to know my colleagues better, and have made a couple of true friends.

-Satisfaction knowing that you're doing something that matters. This goes with the faith one, above. It is deeply satisfying to me that I am helping students to learn things that will matter (even if they don't think that they will right now).

-The "ah ha" moment. There is nothing like blowing your students' minds. It's so much fun to take ideas that they think are set in stone and show them another point of view that completely changes the way that they think about things. To see the gleam in their eyes, watch the wheels spinning furiously at that moment, and know that you caused it are amazing.

To sum up, teaching is hard work. It has long hours, small pay, little respect, and requires a lot of faith that what you're doing matters. However, I can't see myself doing anything else at this point.