It's a really nice lens but the auto focus is going to be much slower that the 135. I'd recommend switching to back button AF and keeping your other hand busy on the focus ring to "help" and speed up the focus. It'll lock much faster that way.
The 100L'.s af isn't slow, but it has a much larger range with a minimum focusing distance of 1ft to infinity... and it will search in that 1ft range. There is a switch to change it to 3 ft and beyond...
I've used the 100L for portraiture, sports, macro... it is a very good lens.
However... no one describes the bokeh as magic like they do with the 135. So your work product might suffer... but not that much.
I've used the 85L and the 135L for portraits they both excel but I found the 135 a little long and the 85L to be a little slow with af. The bokeh on the 85 was amazing but I didn't love the lens like I loved the 135L. I'm hoping the 100L is close just because I do a lot of handheld work and I find it very light and comfortable. If they came out with a newer 135 with IS I would jump on that.