Originally Posted by wickerprints
I have the 24-70 and I completely agree.
When I was starting out I was really worried about ensuring that I had no gaps in my zoom range and I was also really keen on avoiding overlap as I felt it was a waste. I realise now that I was wrong.
55-70mm isn't much. Most of the time you can move to compensate, the rest of the time you can crop to compensate. IMO it's definitely not worth a £900 lens - that money will be much better spend on other accessories or a different lens IMO. Furthermore, the 24-70 isn't a light lens so you'd also be lugging around that extra weight for not much reason.
I bought the 24-70 when I had a 400D/XTi and was unsure whether I would eventually upgrade to full frame. I did make that upgrade but partly because I had the 24-70; I now think that had I never had the benefits of full frame I really wouldn't have missed them. Had I known then what we all know now about the existence & specs of the 5DII and the 7D, I hope that I would have stuck with crop sensors and gone for the 17-55.
If you do go full frame or APS-H then I would save your cash for then and buy both the body and lens at the same time. In this case, if you're finding that the 17-55 isn't long enough now, you'll probably find the same with the 24-70; I have sometimes found that my subject was moving constantly between the range of my 24-70 and 70-200 which can be annoying. But then again I have no experience of the 24-105 and the consensus when I asked your question was that the 24-70 edges out the 24-105 for IQ and low light use.