I have the f3.5 version and love it. I don't have the f/4 - 5.6 so I can't compare but I do find f3.5 useful (really depends on the shooting you do).
For example doing this sort of thing:
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There are comments about it being soft in the corners and to an extent they are true. Wide open at 10mm the corners on my copy are soft, and even stopping down to f8 they are not perfect (corners are pretty good at f11 on mine). However the corners are good wide open on mine from 16mm to 20mm. On the flip side the image centre is pretty sharp f3.5 across the range, the HSM is really good (quiet), and I find f3.5 good for low light and getting a little blur in the background.