So I take it the answer I gave when you asked the same question 8 hrs ago wasn't plan english?

http://answers.yahoo.com/question/index;_ylt=AqcEK_vN9.zcGy4sB7asXMTsy6IX;_ylv=3?qid=20100111151152AAhdpUV&show=7#profile-info-UAors0qraa

Yes, L-series lenses are better in terms of build quality and do tend to offer better image quality through better optical design and components. A non-L lenses may have a plastic body, plastic elements (lenses) and even a plastic mounting ring. By comparison, an L-series lens will have glass and/or flourite crystal elements in addition to a metal body and metal mounting ring. All of this tends to mean an L-series lens will be larger and heavier than a similar non-L lens. Also, L-Series lenses that allow autofocus are the fastest and quietest focusing lenses Canon makes.

You've asked twice, "how much percent better" and again, the truth is no one can honestly tell you the resulting images are "x" percent better. The answer is subject to individual tastes. What we can tell you is that minor flaws one might see in an image shot with a non-L lens will ten to be reduced when the same image is shot with an L-series lens. All of this means there is an improvement in image quality but again, there's no quantitative value or number than can be assigned to this without getting into a discussion that goes far beyond a plain english discussion. In fact, you'd have to be an expert in physics and/or engineering to calculate the difference. That said, you can usually see the difference with your own eyes. That difference may be very slight and may only be present at the outer edges of the picture. And sometimes you can only see that difference when the image is blown up to poster size.

Here's the bottom line. As you know, L-series lenses are not cheap. But you need to be sure that you have evaluated a lens for yourself before dropping a minimum of $700 on an L-series lens. I'm not telling you not to buy one. I don't regret buying any of my L-series lenses. However, I couldn't justify spending over $5000 on such lenses until I knew exactly why they were better than similar lenses I already had and more importantly, was able to recover the cost of those lenses by selling the images I produced with them. On the other hand, if you have money to burn, buy L's all you like. Just be aware that simply buying an expensive lens does not guarantee better images. There are a lot of people running around witht all the best gear in the world but none of the skill to use it. And these are the same folks you see bouncing from one camera brand to another every time they buy a new camera.