For a supposed improvement over the original 24-105, Canon have done a very poor job (at least with the initial batch anyway). My copy has serious back-focus issues which even AF micro-adjust can't reliably fix; the focus also wanders in AI servo mode, with one-shot being the only way to improve your 'keeper' rate. It's a shame as the optics are clearly capable of producing very sharp images (although not a great deal sharper than the version I). The original 24-105 had a similar focus wandering issue, although that seemed mostly restricted to shooting in low light.
Almost all of Canon's next generation L lenses have been significant improvements over their predecessors, so this is a huge disappointment. Perhaps Canon can fix the focus issues in later batches, but until they do, keep hold of your original 24-105s!
It really says a lot about Canon's quality control when a version II is outperformed by the 11-year-old lens it replaces.