I have a cropped sensor 400d and I bought the Canon 70-200 L f/4 last August - it is a marvellous bit of glass and I have had some great results from it, but in retrospect, I find I use it less and less for rail photography. I'm also looking at the 24-105L because that will give me a better range with L quality optics - eventually I will then add the 100-400 'pump', but not until the exchange rate has improved!
I agree that something with a very wide range is a major compromise, and I find that changing lenses isn't a chore. However, I also find that I decide on either my 70-200 or my 17-70 and stick with it for a session - only occasionally does that end up in a duff shot because I had the wrong focal length.
I also try and use the 70-200 when a special is due because of the extra quality of the glass - I happened to be trying it out for the first time when this came along (
PIC). I have also learnt that different lenses behave optimally in different ways and I am now getting very good results from my Sigma 17-70 (
PIC) which I was initially disappointed with.
So, get something like the Sigma with a range of (15-25) to (70-100) and a 70-200 and forget about buying a do it all zoom as I think you will be disappointed.