I've been using this lens for the past 4 months. The image quality is excellent... very sharp... many of my friends have told me my pictures look very "professional" after I started using this lens. The build quality is "L" standard, and because this is one of the newer models that Canon introduced, it is weather and dust proof (not all L lenses are).
This is the lens that I take when I travel with only one lens. It is very versatile, and its size (670g) is much smaller/lighter than the 24-70mm 2.8L (950g). The focal range of this lens (38-168mm) is very similar to a typical point-and-shoot camera when used on a 1.6 cropped DSLR such as the Rebel XTi. If you're a new DSLR user, switching to this lens from a point-and-shoot should be an easy transition (if you can afford the price).
I recently took this lens to Disneyland, and it performed wonderfully. Both day and night shots came out sharp, and it wasn't too much of a hassle to lug around. On rides like the Space Mountain, I kept the camera in my backpack and placed it under my legs, it was not a problem at all.
The image-stabilization inside this lens is one of the newer generations and provides 3-stops of compensation. This is a major improvement over the 1st generation IS found on the consumer grade 28-135mm lens. Also, the dust-proof build quality makes this lens a better long-term investment than the 28-135mm, as the 28-135mm is famous for sucking in dust.
I can't say enough good things about this lens. If you are tired of your "kit" lens and don't mind spending a little more on lens, this is an excellent choice. Don't go for a mediocre zoom lens knowing you will upgrade in a year or two. Get the lens you want now and have "professional looking" pictures with no regrets. Some people say that "noobies" shouldn't shoot with L lenses because it's a waste, but trust me, I was a newbie 4 months ago and saw a huge difference in the quality of my images after switching to this lens. Now that I've learned more about DSLR photography I appreciate this lens even more. This is a lens I can see myself using for many years and through many generations of DSLR body upgrades. I'm glad I decided to spend more on glass and save on body. Lenses, especially "L" lenses, hold their value very well but camera bodies depreciate in value quickly.