The 24-105L is a good lens, and the best results are at about f8.
Honestly, because I look for better results, I use it only on holidays, or on location to check with focal will be the best, and then I switch to a prime (35f1.4, 50f1.8, 85f1.2II are excellent).
Also with a max aperture of f4, this is a limitation associated with the quite noisy 5D (mark I) sensor after ISO 800.
Everything depends of which quality you are looking for.
To get some idea, check out this gallery I just added, 24-105L + 5D.
http://www.pbase.com/nicolasdechamps/ardecheRegards,