I'm currently using 550D + Sigma 17-70 OS, but I plan to move to a FF body sooner or later (most likely I'll grab 5dmkII when mkIII gets released), and that's why I want to buy 24-105 now and use it on my cropped camera (as far as I know it's pretty good for APS-C cameras), but the lens is 4 years old and has some drawbacks, e.g. pretty pronounced distorsion at 24mm. So is it worth waiting to get it, or I'd better buy it now and save money for an FF body?