Well f/4 isn't exactly "slow" for a zoom - with the exception of the Sigma 18-35, f/2.8 is the fastest you'll get on a zoom lens.
I think the biggest problem the 24-70 has is the 24-105 which is far cheaper and more flexible. It exists in a bit of a vacuum between the 24-70 f/2.8 and the 24-105 f/4 IS.
This is exactly what I'd agree.
The 24-105 is about £200 cheaper brand new and has a longer zoom on the tele end. It's also long been sold as a 'kit' lens on Canon's higher end DSLRs, so there are an abundance on the second hand market. It was the first L lens I ever bought.
With the 24-70 focal length, if you need f/2.8 then you wouldn't hesitate to buy that lens, its very expensive, but its a workhorse for Pros and offers the best optical quality if you can afford it.
If your budget is lower, you are going to be looking at the f/4 Ls (or perhaps video for IS) then I think for the majority the 24-105 is more appealing.