I'm trying to decide what to get. Why did you pick this one?
When deciding on lens, think about usage. This lens goes from wide to super telephoto, its primary benefit; it is not ultra-sharp (primes), fast (other L glass is faster), or light (50mm f/1.8 prime is pretty cheap and tiny). Your first thought is focal lengths needed, followed by speed and sharpness. This is the only Canon "do everything" lens, great for what it is, but also having certain compromises.
The likely usage is travel (when you can't take a lot of specialty glass, throw the 50mm prime in the bag as well for low light), weddings (one camera body and no assistant, but watch your low light situation; better off with two bodies and faster lens, but in a pinch...), and some sports situations (wide to tele in an instant). With digital bodies getting more and more sensitive sensors, super fast glass is getting to be less of an issue. Also, the IS in this lens is pretty good, making handheld telephoto a possibility, rather than an exercise in cry.
I take this lens when I don't want a huge kit with me. Mated to an EOS 5 Mk II, or the soon to be released Mark III, and it is plenty good for around-town shooting. Just be aware that it is heavy, 3.5+ lbs, so working out is recommended. The lens isn't my choice for night shots, and a Canon 50mm prime is very sharp, aside from being ultra-cheap -- there's no reason not to have one of those in your pocket at all times.