I had a 35-350L for a time as I wanted something like the 100-400L but with more on the wide angle end. On the long end wide open, it seemed decent compared to the 100-400L but it did lack a little less sparkle. Not sure how to describe it. It didn't feel soft, but still somehow less clear.
I didn't keep my sample though, fortunately obtained through a dealer so I could easily return it. It was very old and well used, the zoom mechanism was sticking and the AF sometimes went funny too. So make sure to check any used samples thoroughly before buying. I'd definitely consider another one if I saw it in good condition and price.
Canon 1D, 300D IR, 450D full spectrum, 600D, 5D2, 7D, 7D2, EF 35/2, 85/1.8, 135/2+SF, 70-300L, 100-400L
EF-S 10-18, 15-85, MP-E 65, Zeiss 50/2 macro, Sigma 50A, 150 macro, 120-300/2.8 OS, Samyang 8mm fisheye