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?
I would wait until you go FF. That way you might at least get it at kit discount price perhaps or maybe something better will be out by the time you finally go FF.
Personally I didn't think it was all that hot on a 5D2, although the IS, AF, range are very nice to be sure. I doubt it would really even work any better than you Sigma 17-70 OS does for you know on your 550D. And it won't go wide at all, which may be a huge negative for you for now.
My Tamron 17-50 was sharper on my 50D than the 24-105 (although the 24-105 wasn't bad on APS-C at all, just not quite as crazy super sharp if you looked really closely, on FF I didn't care for the mushy edges, CA, distortion on the wide side, you might need to pony up for the new 24-70 II if you want a zoom in that range and expect edge to edge goodness though on a high MP FF or look to the 24 2.8 IS prime or something). Even my Tamron 28-75 2.8 actually did a little bit better on FF, other than having a little bit worse contrast. It also has f/2.8, although the AF is slower, it doesn't have as much range and lacks IS.
That said some people do swear by it. I hated it though.
It's a weird lens in that it seems to be one of the lenses that most often appears on lists of people's favorite/best FF lenses and it is also perhaps the lens that most often appears on lists of people's least favorite/worst FF lenses haha. Not sure what to say. Maybe lots of copy variation. Maybe lots of differences in the way various people tend to shoot. Maybe lots of differences in people's expectations. Maybe all three. Canon's own MTF chart does appear to show terrible edge performance on FF though, much worse than that of the 24 1.4 II (or the upcoming 24-70 II and 24 2.8 IS) or the Zeiss 21mm.