Might be a great compromise...

As long as you don't over-pay. They aren't rare!
I got mine just last week to use on my 6D. Apart from being small and light and mechanically nicely-made, delivering excellent AF speed and accuracy, here is the essence of what you will get optically:
Very good sharpness across most of the image field, right from open aperture. This holds true at all apertures, as far as I can so far tell. I am absolutely not seeing an improvement when stopped down, except at the borders and corners. But most of the time, this is a lens you can be confident of using at open aperture, because contrast across most of the field is already high. The colours are typical good Canon.
The corners never get truly sharp, at least at wider focal lengths, but I think they are about the same as any wide zoom from this era. The corner performance reminds me of, say, the 10-22 for 1.6 crop. It's not horrible, but it ain't great. 
Borders mostly look reasonably good. 5.6 will help with this.
Distortion is fairly high at 20mm but lowers as you raise the focal length looks lowish for most focal lengths much above about 24.
Vignetting and CA are mostly taken care of for my JPEGs by Canon's correction profiles.
It doesn't flare as badly as I feared it might.
I am confident a 17-40 L is going to be better in borders and corners, but it's a fair bit bigger and heavier, and I got my 20-35 for about a quarter the price of a used 17-40. I am not in a position to quantify just how much better as I always managed to stop myself buying one. My guess is a 20-35 would be worth trying at up to half what you'd expect to pay for a used 17-40. It all depends how much value you place on small size/weight, and how much you want to spend...