I've been shooting with the newish Canon EF 24-105mm f/4L IS USM for a couple months now and feel comfortable reporting on its performance in the field.
The 24-105 lens (18 elements in 13 groups) has a constant aperture of f-4 throughout the entire zooming range. The image stabilizer is Canon's latest generation, and it really does enable you to shoot at shutter speeds that are 2-3 stops longer than without the stabilizer. Since I prefer to shoot "existing light" style when working in bright interiors, the IS comes in very handy. For example, I'm normally apprehensive about hand held speeds that dip below 1/30th of a second. With this lens I've captured good shots at 1/8th of a second with no visible camera shake. Coupled with the fact that the lens performs well wide open at f-4, you can actually use this zoom for low light photography.
Additionally, all of this performance is squeezed into a package that is remarkably compact (3.3 in. x 4.2 in., 23.6 oz. / 83.5mm x 107mm, 670g). This lens feels much lighter and more balanced than the 24-70mm f-2.8 L lens (3.3" x 4.9", 2.1 lbs. / 83.2mm x 123.5mm, 950g). The mechanics and zooming feel solid (5-group helical zoom), as you'd expect with a Canon L lens. And the package includes both a lens hood and a soft leather-like pouch.
The front filter ring is 77mm. I recommend that you use the "thin" filters so you don't incur any vignetting at the wider focal lengths. Since the front glass element is large, you should probably factor in the price of a protective Sky 1A or filter of your choice to leave on at all times.
The USM zooming motor (inner focusing system with focusing cam) is ultra quiet and very responsive. I think Canon L lens are the quietest of any autofocus lenses I've tried.
As for the images the lens produces -- sharp, contrasty, and saturated throughout the zooming range. This is a lens that performs well on the Canon 5D, and if you don't mind the 1.6X magnification factor, is a good choice for the APS sensor cameras too such as the Digital Rebel XT. The only real optical tradeoff I've noticed is that the 24-105 can only close-focus to 1.48 feet (.45mm). So this is not a lens you would use for your close-up photography.
Is it worth the hefty $1,250 price tag? That depends. If you like having an all-around zoom that you can trust to cover most shooting situations, including some existing light, I think so. I like the IS functionality, constant aperture, and relatively light weight compared to other "L" zooms. I recommend this lens for serious shootings looking for an all purpose zoom.