A must do while in Paris, even if just to say "I went up the Eiffel Tower." Yes, you can take the stairs; my sister opted for this option and it is a good one if your legs will hold out, as it is less expensive. Or do like the rest of the lazy Americans and take the elevator! You will have to wait in line for tickets either way, and the shortest line of the four days we were there was Sunday morning.
The elevator will take you first to the middle landing where there is a place to eat as well as an observation area. You then hop on another elevator to take you to the uppermost level. This is where you can get the views of Paris! I had never realized how large a city Paris was until this. From up here you can pick out most of the major sights, and also see the transition from Old Paris to the newer business districts of Paris.
The Eiffel Tower is really much larger and taller in person than I would have imagined. The elevators are an experience themselves, as they travel at an angle, and you get a view of the inner workings of the construction, though I would guess that taking the stairs would provide a better view of this.
I was told the restaurant on the first landing is rather expensive, but there was such a crowd there that we never could find out. There is also a restaurant at the base of one of the pillars; I grabbed a peak at the menu and it was incredibly expensive (in the range of 30 euro per person and up). The price for the elevators was €12 per person, though we did not have to pay for our 1-year-old.