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Is it a good idea to use Canon 24-105mm f4 L IS USM lens, that is made for FF sensors, on APS-C sensor (like on EOS 400D or EOS 50D).

I have 28-105mm f3.5-4.5 USM II lens, but thinking of buying 24-105mm L lens. I do not have (and probably will not have in next 2 years) FF sensor camera.

Is it a good idea?

yeah, but 28-105 is cheap lens (250eur), and 24-105 is L glass (1100eur)!

how much sharper is that 24-105 L comparing to 28-105.

Most important things are: sharpness, precise focusing, and low chromatic aberations (prurple fringe).

Is it worth to give about 1000$ (i would sell my old one) for that lens, comparing to 28-105 non-L glass?

I know that, I am ok with 1.6x crop factor.

I am just wondering is it worth to pay 4x more money for slightly better lens? Or that lens is really that much better?

Peter, in this site
http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?FLI=0&API=0&Sample=0&FLIComp=0&APIComp=0&LensComp=0&CameraComp=0&SampleComp=0&Lens=355
you can compare 2 canon side by side in every focal lenth and aperture... unfortunatelly they dont have a 28-105 to compare with the 24-105

Well at 24 and wide open perhaps but compare it with the 24-70 f2.8 at 24 f4. Not really a great difference.
And compare it at 50mm f4 with the 50mm f1.4 That is a difference

I was a fan of the 24-105 until I saw many tests and compared with 24-70, including the tests @ the-digital-picture.

Now I clearly see, for the best results I need the 24-70. Who cares about stopping down the lens? I need an f2.8 lens because I want to use at f2.8. For stock. With the today's 15-20mpx cameras I'm happy if the zoom lens draws me a true 15mpx image. The 24-105 is far away from this at the widest aperture, except the center.

All lenses gives CAs. Take a look at the 70-200 2.8 Is an expensive glass but it is a lab test where they are exagerated. In real life photos you can live with that CAs I think.
Take a look those red CAs in the 50mm f1.2 (1500€ glass) too

I have the 24-105 F4L and use it a lot ... on a 20D

It depends really what you want to shoot.

Its a great (convenient) walk around lens due to the focal length and weight.

For "standard stock" (f = 35 -100mm + studio strobes + F8 and higher) it probably is overkill. get a good copy of the 28-135 IS if money matters.

CA at the wide-angle is quite strong. Especially in bright sunlight on high contrast lines in corners.

for weddings 2.8 is nicer, but it works ... or use primes.

No IS in the 24-70, they are talking about a new 24-70 with IS but just a rumour.

I have EOS 400D and 28-135mm IS (and 50 f/1.8 which I uses a lot for isolated objects).  I've been considering the 24-105 IS as an upgrade, for better outdoor images etc.

Is the consensus that it's not worth the money to upgrade to 24-105 for stock images??

According to the comparison tests on www.the-digital-picture.com, the 24-70 lens does seem to have better IQ than the 24-105.  The price difference between the two (when not purchased as part of a 5D or 5Dmk2 kit) is only $100 here.  It's really hard to decide whether it's better to get the lighter 24-105 with extra 35mm and IS or better IQ with the 24-70 and f/2.8....