I like you was in the same pickle, thinking of going full frame but wasnt sure when. The 24-70 F2.8 is getting long in the tooth with no IS, it is expensive although a fantastic lens, the 24-105 F4 IS would be a great lens but i feel F4 isnt fast enough and having the 2.8 really means the lens is available for use in nearly all situations. What really put me off is a standard lens should be wide and a medium telephoto, these two EF lenses are pretty much medium telephoto, with the range starting at 38mm on a crop camera. I dont like the idea of changing lenses every time I need wider than 40mm!! I use the wider spectrum more than the longer spectrum so the EF lenses were not for me.
I have always wanted the 17-55mm F2.8 IS, it has been rated so highly just didnt get round to taking the plunge. I had the 17-85mm while the optics wernt the best and the variable aperture made it a pain in lower light it was a good lens and served its purpose. I Finally decided to take the plunge and got a great 3 month old second hand 17-55mm which i paid Â£550!!! I saw it and thought this is too good to be true so snapped it up immediately!!! (London Camera Exchange - so you know its good) Sold the 17-85mm and the optical quality of the 17-55 is incredible! I have read that the 17-55mm is basically an L series lens without the weather sealing, and i cant agree more and wont look back. It is so sharp at F4 (bit softer a 2.8 but that is to be expected im not a pixel picker) and the boeker is very pleasing, I love the 2.8 makes this lens such a good all rounder.
Like previous posts have said I definitely wont loose money on this one! Since then I haven't seen one for less than Â£650 so i was very lucky. Although it didn't have a hood because for some reason Canon don't provide a hood with any of their EF-s lenses, although they are as expensive as the L's. But it was boxed and looked like it had shot less than 500 frames. The guy at the shop said the previous owner had gone crop but decided very quickly he wanted full frame so sold all his EF-s kit ergo bargain for me!
If i have one problem with this lens... it would be the length, being used to having 85mm to play with going back to 55 feels a little too small, but i just need to get used to it, also slightly disappointed in the minimum focusing distance. Unlike most standard zooms they usually have a macro distance of 0.25-0.35 but this isnt rated with any macro ability, there have been times where being closer would be nicer especially seen as tho the zoom is quite short. But i wanted to get the new 100mm macro IS so im not too worried. (maybe i ask too much from the glass, they have made a great job with it as it is).
The small problem i now face is filling the gap, i have the 70-200 F2.8 L and that works out at 112-320mm and there isnt really a lens to fill the 55-110 gap. All the lenses in between either dont fit the bill, are reduced quality cheaper mid range zooms with variable apertures...
In my opinion there isnt a perfect balance but its just something I have to deal with.
For me the perfect lens would be either a 17-70mm F2.8 IS EF-s or a 17-105 F2.8 IS EF-s with minimum distances of 0.25 it wont happen but i would snap at a lens like that.