Rasmus has it right.
I don't think I've ever lost more than $150 on an L lens. I buy a new 70-200 f/2.8 every couple of years because the cost is of ownership is negligible... given how much I rely on that lens.
I have one Sigma that I took from a friend because of it's extreme width. (12-24) - It's fine for that purpose. However, about 8 years ago, when I purchased my fist Canon D30 SLR, I went with a couple of Sigma. Realizing I didn't like them, I sold them within 3 months... I LOST MY ASS. They barely retained 50% of their value.
Zooms are very tought to make while retaining speed, color and sharpness. You need to keep in mind that to make the range of zoom, it requires "elements".... (additional lenses). They add together to make the range you need. As you move them closer or further apart, they change the magnification. So, the more range... the more glass you are shooting through. This adds softness, color fringe and slows the whole process down.
The 70-200 is one of the finest zooms in the industry. The later f/4. version is affordable and quite good.
JT
www.johnthawley.com / www.automotivephoto.net